In reply to Mike Conlon:
I suggest the following:
1. Commission the services of a structural engineer
2. Make an application for building regulations compliance
Unless you have a paper trail, you could have issues if you ever sell up.
When it comes to the weight (mass) of the roof; it's probably 'relatively' insignificant compared to the wind & snow loads which might occur.
Do it right; follow steps 1 & 2 above - it'll probably cost you less than a monkey. Which is perhaps what you'll feel like (or worse) if you get it wrong!
Also; if you are actually carrying out any work (at all) on the party wall; you will (legally) have to serve a party wall notice on your neighbour. You can serve this notice yourself or appoint a party wall surveyor; which might cost you £500.
I'm sorry, I can't give you the answer that you really want, but I offer the above advice for free!!