So i’ve been given (chakaping – thanks!) An older set of SLX brakes 665s like these and wondering if they are worth refreshing or buying new?
Ill have to get a longer hose for the rear + pads at each end.

If they work at the moment, fine. Putting a new hose on is straight forward and cheapish. However if there are any problems I would bin them and go new. I have two bikes with the same Slx, one took a new with no problems, the other was always losing pressure and needing bleeding so I dumped them for new Deore which are excellent. Only minor snag is I now have different pads front and rear.

CRC do SLX for £48 per end. I am estimating that this is not far off the cost of refurb, assuming that you planned on using genuine shimano parts

^^^ yep via LBS so guessing its going to be similar cost 🙁

Other than replacing the hose and pads and perhaps giving them a bleed, theres nothing else you can do to “refresh” them.
No spare seals available etc etc.

I have both 665s and the newer ones (666?) on bikes and they’re both great. If you’ve been given a free pair Id go with that…