Hoping someone can help....

I recently bought my A6 C6 Allroad (2006) 3.0tdi and developed the fuel level fault. I had this fixed at a local indy - new fuel level sender etc. Anyway a few days after getting it back I'm getting the P1020 and P0088 fault codes along with amber flashing glow plug light and limp mode. On one occasion I've also had the amber engine light come on.

I thought it might just be one of those things so I tried clearing the codes but no joy they have reappeared along with the associated warning lights. I do want to add that I have every confidence in the garage who did the work and that I realise the new issues could be unrelated to the work done but also thought is was worth mentioning to help form a full picture of the fault.

The vehicle does also have fault codes for the battery and the glow plugs although from what I can gather this wouldn't cause the fuel related codes. I've listed all codes below:

OBD2:
P0088 (pending) - Fuel Rail/System Pressure - Too High

Body/convenience system:
01135 - Interior Monitoring Sensors
01134 - Alarm Horn (H12)
01134 - Alarm Horn (H12)

Engine:
P161B
P161C
P161D
P161E
P161F
P1020 - Fuel Pressure Regulation: Control Range Exceeded

Any help on where to go next with this would be very much welcomed........