If the bounce test shows no weakening of damping effect, i'd be inclined to give the underside a wash down, in particular the shockers in question, and then recheck them after a few weeks running to see if oil misting is still an issue...if the dampers are still damping sufficiently one could clean things up just before the MOT if one were definately selling.

If they're getting weak then its a no brainer to swap any weak ones (plus its opposite) asap and enjoy the safer handling yourself, depending on how much this costs could have some influence whether you sell or keep i imagine.

Should i just leave these untill i have to change them? Should I replace the shockers( all 4?) , is it worth it? What would be the £cost of doing all three or even four.

I would have thought that shock absorbers should be replaced in pairs ie both at same end/same time.