If you are going to the trouble of taking the sump off, for an extra 30 quid or so, I would change the oil pump anyway. New pump, new oil, new filter...should sort the problem.
But first, remove the oil filter and very carefully cut it open. Remove the card consetina element inside and stretch that out. You are looking for bits of hard debris, white metal and iron filings. The iron filings may be a knackered pump in which case, spending the money on a new pump is worth the gamble. But if you find flakes of white metal too, that means that the iron filings have been eating away at your bearings and worn them out, causing your low oil pressure.
If there are none of these things present, then change the pump. You should be OK.
