Thanks for the replies guys, it sounds as though this could be an interesting thread!
I am aiming to complete the brighton HM in under 1:50 if possible. This is my first competetive run of this size (I have run 10ks before), so I will learn how to approach this on my 10 mile practice run in January.
Unfortuatnely at Christmas I have to visit lots of family members and we usually have a focus around dinner (As with all families at christmas I expect!) - I think what I will do is dish myself a plate, and stick with that, no seconds, no extras, just what I have chosen and accounted for. Desert is an exception, its christmas and I am going to have som pudding darnit! 
Lardarse - congraulations on your marathon, it must have taken alot of hard work and determination. I too would like to complete one, possibly towards the back end of 2013, if not early 2014 as a marker.
Wilkie - I too have been eating at a defecit over the past few weeks to 'compensate' for the bigger days of eating and enjoying myself. So far in december I have run up a defecit of 6500 cals (through less food and excersise), and this will more than adequately line me up for a net zero once christmas and new year have been dealt with!!
On a serperate note, is 141lbs at 5ft 10 too light? - I know runners who weigh 170 lbs at the same height and are just fine