Once they install a modern master socket their responsibility ends at that socket. Any internal wiring they might have put in years earlier becomes yours and is your responsiblity. You would not expect the previous houseowner to remain responsible for mowing your lawn?
I'm unclear why you need a power socket by the phone master socket, nor does your phone need to be in the cupboard.. You can take a phone cable extension (like this kit if you are not handy :
http://cpc.farnell.com/pro-signal/31098r/telephone-extension-kit-10m/dp/TE05887?st=telephone%20extension%20leads) from the phone master socket to wherever you like. In my experience though the master socket position, and the line to it is often inconveniently placed or ugly, and I am "always" moving them. Eg I recently bricked up a doorway that had the lead going through the wooden frame; nor do I like leads fixed to my house facia with rusting staples.
Consensus on the web is that Open Reach engineers are not very worried unless you do it wrong, or with wrong fittings (they can tell wrong wiring from the exchange), and they are not going to crawl round you attic to check you have not put in a new junction box for example. The master socket must remain easily accessible though.