My '82 RS125DX has 5 loose spokes on one side of the rear wheel. While it's not affecting the ride I'dlike to fix this and it may possibly be an mot fail? I've tried to adjust them but the adjusting nipples are rusted solid. I've tried a bit of wd40 and 3 in 1, clamp the spoke with mole grips close to the nipple and worked them backwards and forwards but no joy.

Any hints or tips how I might free these?

Yes it could/should fail the mot.

The loose spokes will cause some kind of deformity in the wheel as their not doing their share of the work.

You cannot replace single spokes either. Its rebuild time.

Yep as above rebuild time I'm afraid BUT you can now use some nice shiny stainless steel ones..!

Thanks for the replies. Is there any reason why I couldn't remove the wheel and then tyre, cut out just the offending spokes and replace them, rather than face the cost of a complete rebuild?

You have to put the spoke in the hub first, then move the rim around to get the spokes in. 

I suppose you could undo every spoke removing the nipples, then fit the new spokes then retighten and true everything.

way too risky, don't know how you can value your life against a few spokes

you want a wheel collapse when the wheel is under load? (braking, corners etc)
 

you can do this yourself using youtube vids (I did) get spokes from thailand as I and John have in the past