350 aint far off,
paint will be roughly 40 quid then primer looking at 30 quid, and laquer 20 quid, then materials like wet and dry say 15 quid on that, thats 105 quid.
then labour and they will take in to account ware and tear on equipment, then most places will charge per item, so skirts will be 100 quid, then 75 for each splitter, and 50 for mirrors, so that is 300 for all of that, then add on the paint etc u looking at 405 quid.
so personally i would say 350 is a good price, and with that i would check each item before i took it from them to check for quality of work, so if it was bad they would have to redo it, ask them for a written quote first with a brake down of costs.