Here's a summary of my issue

Ran low on fuel a week or so ago but forgot to refuel on way home. Next morning showing range of 15 miles. Started OK but cut out at idle twice on a 4 mile journey to fuel station. Range showing 10 mile at fuel station. Half filled tank and started OK again but cut out at idle three further times between fuel station and destination 3 miles away. This was at 9am. Came out to it at 1PM and would turn over but not start. Briefly started twice but only ran for about a second each time. Battery ended up flat. Battery partially charged but generic OBD2 reader which has previously worked on it would not connect. Display showed ESR Fault/ Auto parking brake fault. After fully charging battery it still wouldn't start but only showed Auto parking brake fault now.

Called out mobile mechanic who diagnosed a faulty in tank fuel pump. Came out today to fit it and after spending two hours, what seemed like "p***ing about" and looking for the fuel pump, which was not under the back seat as expected but actually in the luggage compartment (which I found myself after I decided it HAD to be somewhere!) and the fuel pump relay (which doesn't appear to be in the luggage compartment as it shows in various places online?) came to the conclusion that there is no power to the fuel pump connector, however, there WAS power to the fuel pump fuse.

He has just come back to me with a message saying that "my old boss says that on a A6 C6 there is also a very common issue which is a ignition start relay, which is very likely that given your staling and then cut out, also no power to pump and suggestions that the evu has gave up, My old boss has just looked into it for me, Under drivers side dash small black relay"

Does that sound plausible?

Any and all suggestions gratefully accepted - I will be taking my meter in with me tomorrow and will try and trace things as far as I can, however, having some idea of where to look would be much appreciated.

Thanks in advance