More probs with my Mk4... The oil pressure light is taking longer and longer to go off

It started on Fri when the oil light came on at a visit to the services after a long run (2 1/2 hours) but went out after a hundred yards and over the weekend it's been taking longer to go off.
Seems like when starting the engine it takes a while for the oil pressure to build, or the sensor to switch the light off - it's now on constantly but as I need to get to work etc. I've no choice but to drive on.
I'm hoping that it might be the sensor - if it's the oil pump them that would be the last straw and it'll be off to the breakers yard for the ol' gal.
Can some good folk confirm the location of the sensor on a 2.2 VTEC Mk4 (above or below the filter) and is it easy to replace - thanks.
Anyone else had a oil warning light do this? If so, what was the problem (and cure)?
Thanks,
Dobbs.