I just had the very same thing with the flexi section. To install a 4-2-1, sports cat and 2 new O2 sensors (after an hour 'arguing' with the old ones), Mike Satur billed for 4hrs, so just to do a flexi pipe, can't think it'd be more than 2 at the outside.
I think a down-pipe and flexi was about £150 ish, but that was only noticed in passing....no idea if there's cheaper alternatives.
And while hideously expensive, I can't recommend the 4-2-1 and sports cat highly enough.... Totally transformed the car!
Edit: and I've been quoted £40 - £45 per hour at various MG specialists