Rear brake won’t lock up wheel under hard braking so I presume it contaminated so just wondering what needs changing

Just pads.

Clean rotors with Isopropyl alcohol.

what are the brakes? I had a similar problem with Deore brakes I hadn’t ridden for a while and so changed pads and discs plus put new fluid in. After 1 ride back to square one – apparently the seals leak after a while and spray fluid on the pads/disc. New brakes time.

If you suspect any kind of oil/ brake fluid contamination replace the old pads and give the calipers and discs a really good cleaning with hot soapy water and you will be fine I’d suspect. A leaking seal is an unusual event in my experience. What makes you think that there is any contamination? TBH, give the brakes a good bleed before anything else, then try new pads.

Hose wasn’t long enough for change frame so changed it. Took pads out but spilled some oil in calliper Guessing it’s got on rotor then pads

Assuming you did a decent bleed it could just be that you need news pads and that they need bedding in. Some people have had some luck blowtorching or otherwise cooking contaminated pads and bringing them back to usefulness but it’s never worked for me.