Wow Jellybean, you have been busy on forums :-) Hopefully you have by now received some good advice:
http://forums.moneysavingexpert.com/showthread.php?t=4925320
I wish the land registry had been a bit more helpful for you, so it looks as though contacting your solicitor might be a good idea to check a few things. Apart from the practical side of things, you will definitely need to check with your mortgage lender to ensure they would be happy with you agreeing to the deed of easement and that there are no hidden charges. Also, if you are first time buyers - don't underestimate the knowledge of your parents - ask them :-)
Although the builders will try and put things back to normal - will they? What if the works over-run? What if they do a rubbish job - make sure you have something in writing to ensure that you are covered financially if this happens - again, something to a legal-eagle.
Ask the Mortgage lender if the value of your property will go down if this work is done.
And lastly - If you do go ahead - the property developer will literally 'up S*** creek' if you don't agree - so hold tight until they up their offer considerably - remember, you have All the time in the world - They don't!
All the best with whatever you decide to do, money is money, but your house is YOUR home :-)