I’ve an old set of XT (9 yrs old) brakes which have the leaky seal problem. Must be very small as you can’t see it but it’s there. Same for the SLX but not 100% sure.
Is there much I can do to repair/save them ?

No. But if there’s at least 2 good calipers out of those 4, you could swap them around to make 1 usable set. If not, they are probably too old to persevere with/buy new calipers for.

I’m pretty sure then XT has a tiny leak as everytime I’ve fitted them the rotors are contaminated and squealing everytime. The SLX ones were fitted in an effort to resolve the squealing on the bikes tainted with the XTs so they may have just picked up the contamination.
Is there anyway to test or identify the leak ? presume it’s the seals.

Would older levers work with new calipers ok ? Might just get some new calipers and ask the lbs to swap them.

Leaky deals ? ?
Any idea how I can find the leak, or which seal (or deal) it is ?

Oh, in that case properly clean the rotors then re-try the slx ones with new pads.

Do the xt calipers look dusty? That’s often a sign as the fine oil spray attracts dust.

Shouldn’t be any trouble fitting older levers to newer calipers. There is a fitting (‘end’) revision in the Shimano range – pretty sure it was the M615/M675/M785 series that saw the new spec when servo wave happened. TBH, if you’re happy bleeding brakes you’re perfectly competent to change a lever. Just don’t try to reuse the olive.