diff --git a/lambda/orc_ingestion.py b/lambda/orc_ingestion.py
index 49b9ff0d..ea089726 100644
--- a/lambda/orc_ingestion.py
+++ b/lambda/orc_ingestion.py
@@ -1,8 +1,11 @@
 import json
 import logging
 import os
+import re
 import uuid
+
 from datetime import datetime, timezone
+from typing import List
 
 import boto3
 
@@ -13,83 +16,88 @@
 s3_client = boto3.client("s3")
 
 
-def lambda_handler(event, context):
-    """
-    When using this lambda, please include previous_ods_code in the event json.
-    Example:
-    {"previous_ods_code": "ods_code"}
-    """
-    try:
-        previous_ods_code = event["previous_ods_code"]
-    except KeyError as e:
-        return {"statusCode": 400, "error": "missing param 'previous_ods_code'"}
-        
-    file_to_ingest = os.environ["INGEST_FILE_NAME"]  # this env var is set as "Patient-List-Test" in terraform
+class DuplicateNhsNumberException(Exception):
+    pass
 
-    ingestion_bucket_name = os.environ["S3_BUCKET_NAME"]
-    suspension_queue_url = os.environ["SUSPENSION_QUEUE_URL"]
 
-    nhs_number_list: list[str] = get_nhs_number_list_from_s3(filename=file_to_ingest, bucket_name=ingestion_bucket_name)
-    all_trace_ids = []
-
-    for nhs_number in nhs_number_list:
-        trace_id = new_uuid()
-        all_trace_ids.append(trace_id)
-        logger.info(f"Assigned a new traceId: {trace_id}")
-
-        suspension_message_json = build_suspension_message(
-            nhs_number=nhs_number,
-            previous_ods_code=previous_ods_code,
-            nems_message_id=trace_id,
-        )
-        send_message_with_trace_id(
-            message_body=suspension_message_json,
-            queue_url=suspension_queue_url,
-            trace_id=trace_id,
-        )
-        logger.info(
-            f"sent message for a patient to queue with traceId: {trace_id}"
-        )
+class InvalidFileFormatException(Exception):
+    pass
 
-    logger.info("Here are all the trace ids that related to this ingest:")
-    logger.info(str(all_trace_ids))
 
-    return {"statusCode": 200, "body": str(all_trace_ids)}
+class InvalidNhsNumberException(Exception):
+    pass
 
 
-def get_nhs_number_list_from_s3(filename: str, bucket_name: str) -> list:
-    local_file_path = f"/tmp/{filename}"
-    s3_client.download_file(
-        bucket_name, filename, local_file_path
-    )
-    with open(local_file_path, "r") as f:
-        nhs_number_list = f.read()
-    os.remove(local_file_path)
-    return nhs_number_list.split(',')
+class NHSNumberValidator:
+    @staticmethod
+    def validate(nhs_numbers: str) -> List[str]:
+        if not bool(re.match('^[0-9,]+$', nhs_numbers)):
+            raise InvalidFileFormatException('File should only contain numbers and commas.')
+
+        nhs_number_list = nhs_numbers.split(',')
+        if not all(len(x) == 10 for x in nhs_number_list):
+            raise InvalidNhsNumberException('All NHS numbers must be 10 digits long and there must be no trailing commas.')
+
+        if len(nhs_number_list) != len(set(nhs_number_list)):
+            raise DuplicateNhsNumberException('Duplicate NHS numbers found.')
+
+        return nhs_number_list
 
 
-def new_uuid() -> str:
-    return str(uuid.uuid4())
+class S3FileReader:
+    @staticmethod
+    def read(filename, bucket_name) -> str:
+        response = s3_client.get_object(Bucket=bucket_name, Key=filename)
+        file_lines = response["Body"].readlines()
 
+        if len(file_lines) > 1:
+            raise InvalidFileFormatException('All NHS numbers must be contained on a single line, separated by commas.')
 
-def get_timestamp():
-    return datetime.now(timezone.utc).replace(microsecond=0).isoformat()
+        return file_lines[0].decode('utf-8')
 
 
-def build_suspension_message(
-        nhs_number: str, previous_ods_code: str, nems_message_id: str
-) -> str:
-    return json.dumps({
+def process_nhs_number(nhs_number, previous_ods_code, suspension_queue_url) -> str:
+    trace_id = str(uuid.uuid4())
+    logger.info(f"Assigned a new traceId: {trace_id}")
+
+    suspension_message_json = json.dumps({
         "nhsNumber": nhs_number,
-        "lastUpdated": get_timestamp(),
+        "lastUpdated": datetime.now(timezone.utc).replace(microsecond=0).isoformat(),
         "previousOdsCode": previous_ods_code,
-        "nemsMessageId": nems_message_id,
+        "nemsMessageId": trace_id,
     })
 
-
-def send_message_with_trace_id(message_body: str, queue_url: str, trace_id: str):
-    return sqs_client.send_message(
-        QueueUrl=queue_url,
-        MessageAttributes={"traceId": {"DataType": "String", "StringValue": trace_id}},
-        MessageBody=message_body,
+    sqs_client.send_message(
+        QueueUrl=suspension_queue_url,
+        MessageAttributes={
+            "traceId": {
+                "DataType": "String",
+                "StringValue": trace_id
+            }
+        },
+        MessageBody=suspension_message_json,
     )
+    logger.info(f"Sent message for a patient to queue with traceId: {trace_id}")
+
+    return trace_id
+
+
+def lambda_handler(event, context) -> dict:
+    try:
+        previous_ods_code = event["previous_ods_code"]
+    except KeyError:
+        return {"statusCode": 400, "error": "missing attribute 'previous_ods_code'"}
+
+    file_to_ingest = os.environ["INGEST_FILE_NAME"]
+    ingestion_bucket_name = os.environ["S3_BUCKET_NAME"]
+    suspension_queue_url = os.environ["SUSPENSION_QUEUE_URL"]
+
+    nhs_numbers = S3FileReader.read(filename=file_to_ingest, bucket_name=ingestion_bucket_name)
+    nhs_number_list = NHSNumberValidator.validate(nhs_numbers)
+
+    all_trace_ids = [process_nhs_number(nhs_number, previous_ods_code, suspension_queue_url) for nhs_number in nhs_number_list]
+
+    logger.info("Here are all the trace ids that related to this ingest:")
+    logger.info(str(all_trace_ids))
+
+    return {"statusCode": 200, "body": str(all_trace_ids)}