Thanks for your advice, gents. I've had the piston inverted and soaking the rings in acetone for 3 weeks but no luck on easing them. Heating and cooling was no help either so I'm using a small cutter disc in a small 12v hand held drill device to cut out a piece of the upper ring. I'll let you know how I get on. (It's tricky.)
Nev, re the "big cut in the oil ring groove" that's just the shadow of the land above. My apologies for the late reply but very now and again I get chucked off this site and forget both my name and password! It always causes me grief and delay.
Regards from MikeN.