Thanks for the replies chaps!
As Bill mentions, the spares company rings are deeper than the originals, which are only available in STD, +.020" & +.040" sizes.
It did cross my mind to machine the piston to accept the +.040" rings (as I had already purchased a set) and then gap them to suit. I was unsure that machining the groove wouldn't weaken the piston & if there was a risk of the modified ring being under too much (sprung) tension.
It would also be on the limit of my modest skills, so I would be "putting a job out", which always invloves its fair share of hassle / disappointment.
I'm no expert Howard, but I think +.030" was a standard oversize from the factory, but yes I suppose today it is not a common one - hence my predicament...
Cheers,
Mike