I wouldn't recommend pruning it hard now. I'd leave it in the pot until winter, watering it and using a tomato feed fortnightly. Then in late winter when it is dormant, prune all the main stems back by half to an outward facing bud, likewise shorten all sturdy looking side shoots by half. Remove any dead and spindly looking shoots completely.

When you plant it you should do so in a mild winter spell, not frosty conditions. Be careful when you knock it out the pot to plant it as with container grown roses the roots may still be fragile and the soil ball might fall away taking much of the root system with it. From what you've described about the size it will need 2 people to plant it.

Snap!

Thanks both of you.
I always thought that you could plant container grown plants at anytime. Does this not apply to roses?

Yes you can if you want to, but just best not to cut it hard back, that's all.

Thanks Stera. I can confirm that with the greatest of difficulty I managed to get it in the ground complete today. What an effort but I got there in the end.
I'm kind of pleased I made the effort though as by the time I cut it out of it's pot (easier than trying to knock and tip it out) it was bone dry. Despite the fact that I watered it well on Saturday and Sunday obviously the water just wasn't penetrating the centre of the pot.
I came away unscathed and only one stem snapped in the process.
Thank you both for your input.

Quite scarey that it was still dry after lots of watering - just goes to show how careful we've got to be. Glad its all planted up safely.