You're not going to like this but.. that caliper looks a little the worse for wear as well so first off check the bleed nipple is free to turn - use a ring spanner rather than an open ended one - if that won't turn then you've got another scrap caliper.

Carefully cut the hose fitting just above the hex nut that secures it to the caliper and try using a ring spanner or socket on it - as you've rounded it you may need to co for a slightly smaller socket and hammer it on, you might then be able to undo the fitting.

Then, and this is most important, get a new set of seals and overhaul the caliper, using compressed air to blow out any swarf that cutting the hose has created before reassembling the caliper.

On reassembly bleed the braking system acording to the description in the Haynes manual.

Overhauled calipers appear to be about £40 on e-bay (including exchange surcharge) and that might be a better option looking at the state of your one!!