Well I did it in the end without locking the crank (although neanderthal's pics show it's something I didn't find - it may well be there but hidden amongst all those pipes)!
Applied a bit of common sense and took the cam cover off to check that all the valves were indeed closed at TDC. Once that was ok then it was a case of undoing the pulley bolt which was the tricky bit as there is no resistance without the crank lock. I stuck it in gear with the wheels down and gave it the heave-ho - twisted the head on my 3/8" driver in the process but finally undid it with the 1/2" on the torque wrench (I know bad form) and then just rotated the crank round to the position shown in Biotoxic's guide as TDC. Slipped on the new belt and turned by hand but didn't feel right so I think the belt was one tooth out so off it came, slipped it round one notch and turned by hand again twice. Seemed ok so fired it up and everything seemed good - no bad noises
So then re-assembled the remaining bits - again due to the lack of crank lock, getting the pulley bolt up to 120Nm was tricky but again applied the 'in-gear' trick and all good - surprising how much drag the ancillary belt causes! Flushed the cooling system (undid bottom hose as again couldn't get to the tap on the other side), added 2.5L of anti-freeze + 1L water (assuming what's left inside is water, that's roughly 50:50) as this is all it would take and ran it up to temp - no sign of the fan kicking in and bang on half way on the temp as usual
Now for a test drive and then a rest if all ok
Thanks again chaps