Devizes - very English sounding village
OK, so you are in UK. Good.have you replied to the message from Simon? - on your thread about filters to use.He has offered to send you a book free of charge.This guy is from Ilford.Take up his offer.
2nd. do not get discouraged with problems as once we find out what the problem is you will see how easy it all is, hey, you have developed the film no problems.
As i said in the enlarger thread, i have meopta 6 and the under lens filter did not fit well for me, i do not know if the meopta 3 will have same problem.How about not using the filters to start and see if you can get an image on the paper.
The lens you are using (75mm) is the lens for 120 size film,you should really be using a 50mm lens and i think you will find that the lensboard ( the place you screw lens into on enlarger) is reversible. The lensboard should recess for 50mm and protrude for 75mm.
You say that you are using f 4.5 for the test strips and have tried various times up to 20 seconds, this time at
f 4.5 should have given a result you could work from. 2 seconds would be far too light, 20 seconds should have given some image. I am assuming that you have 35mm negatives by the way.
I suspect you are exposing the wrong side of the paper or your enlarger does not have the correct bulb, or you are not shifting the red under lens filter away when you make your exposure.
And these are all mistakes many still make at times, i have a blackboard in darkroom to list the process and still occasionally forget something.
To test the paper emulsion side theory simply do two test strips at the same time using both sides of the paper.
Assume that this is the paper developer you are using