Before you go the repotting route you could try simply turning the existing pot so that the cactus is leaning away from the light. This may well cause it to straighten up.

As MG has said, you have left it in one direction only for a long time, if you simply turn it, it should self correct with time.

When repotting you need to grip it where there are no spike and repot it that way. Add sand to the soil mix.

That is a very healthy looking plant.

I think it looks nice and querky like that and as it is so healthy I would leave it well alone.

You can make a collar with a folded strip of newspaper to grip the cactus near the bottom. I think it might need two of you to repot it.

I repotted some seriously spiky cacti recently. Wore a long sleeved top and heavy gardening gloves and wrapped cactus in al old pillow case. Worked! If you do repot I would suggest buying cacti compost and adding a lot of fine grit to the compost.

A picture of you togged up in that lot would be a larfe! Lol.

I would place it outside for the summer next year and re-pot.

Lol Cammomile... Bulba possibly did take photos!

I don't know, Chesterfield has a crooked spire and people come from miles a round to see it. maybe you could advertise it as an attraction? LOL

I've tried the turning it away from the light trick - it seems to be the vampire of the cactus world as it leaned away from daylight even more!
Thanks for the advice about the compost/grit mix MG. What proportions would you suggest?

Graeme sorry I should have said either buy cactus compost or use your own compost with 50% fine grit.

Thanks. I'll try the latter as I already have 20litres of multi purpose compost.