Thanks for your reply. My own suspicions and concerns were exactly what you have covered in your reply. In the end the interface was not critical or life threatening and part of a larger system being installed so I went for the beer money.

Since then I have had some satisfaction in knowing that the main system (a complex security system) developed a fault that went unnoticed. My interface worked as it should resulting in the thieves being caught loading their van with £20,000 worth of kit.

How much beer money did I charge for the interface £30 !!!!!!!!

Thanks for the replies,
Harp