Is there anywhere that lists road sportives and similar rides. Looking for one now in the North East or near and thinking about planning to do a few next year where around north Manchester and Guildford may also be a possibility.

Is there a similar thing for mtbing. Is there somewhere that would help plan out some events or do I have to work them out myself?

http://www.cyclosport.org has a calendar on it for roadie events and a (usually very quiet) forum.
BikeRadar and roadcycling.co.uk are both good for Sportives as well.

Somewhere knocking around on here there used to be a link to a calendar run by Numplumz which listed almost every MTB event in the UK…

Otherwise http://www.britishcycling.org.uk is probably worth a look as well.