checked the brakes and they don't appear to be on, when I work out how to change gear I'll try a smaller one, 53x12 might be the problem

You're riding with gears and leaving it in 53x12??
Search the forum for "spin" "spinning" "overgeared" and "knee pain".

I spin when I am in 52 x 12, I am just daaaaaaaammm faaaaaaasssssssssssttttt!!

53x12 is for hill-climb TT's.. I 56x11 on the flat..

and this one for the sprint finish..

Do you have your saddle at a different height on your road bike to your fixed bike?

it feels about the same, fixed cranks are 165mm road ones are 175mm so the saddle on the road bike is 10mm lower, not sure if this is correct though, the seat tube angle is more relaxed so maybe I should tinker with moving the saddle forward a bit

Typically you would drop the saddle height on the fixed by a cm or so.
I don't see how you can be tired riding a geared bike for a while. Once your body has adapted to it the driveline losses are minimal and the gear choice allows you to ride at a more optimal/efficient cadence, ie. once you're used to a road bike you should be more efficient/less tired/faster depending on what you want to do.

My legs used to feel fresher after century rides on fixed, but once I stopped coasting on my geared bike, the playing field was level once more.

It's hard to tell because when doing long rides geared I tend to ride faster down the hills, using up more energy rather than just spinning along. On the fixed though I often think "mmm a nice bit of coasting would be good here" :)

exactly what I was thinking on the last stretch of Dunwich ;-)

saddle position can be quite critical. hippy says on the fixed you need to be slightly lower, which I didn't know about. I think saddle height is usually measured from centre of bb to top of saddle, but as you say, different crank lengths must make a difference because your leg will be straighter.

On the bike that you're most comfortable with, measure the distance from the tip of saddle to the centre of the handlebar. Then use this distance to adjust the saddle fore aft on the road bike. If you do have the wrong distance you'd probably have back discomfort as well as a lack of pedalling efficiency. A 1cm difference can have a big effect.

It was the 50k before the feed station that I was thinking it.. the run to home I was quite excited about so wasn't so bothered. Dunwich!!!! :D

I love riding fixed long distance. You can take a flip flop hub and have 2 different gears, it's great.