"d-i-Why?!" wrote

***Just been quoted £300 parts and labour to replace an old fuse box with
a new consumer unit. Is this a reasonable quote? The parts are only
about £50 but I believe the new part-p regulations prevent me doing
the work.***

Need to check this with your own Building Control Dept but:

If you are competent to do the work yourself it should be possible to:

Advise BCO of work to be done (issue building notice)
Pay fee for BCO inspection (about £70 IIRC for upto £1000 value work)
Co-ordinate for them to inspect/test on completion, but prior to energising!

May differ between councils and it is worth checking that they are capable
of and willing to test.

Phil