As you are in the UK, where nitro is supposedly very expensive, perhaps the new OS 70 Ultimate could work?

Traditionally os's are more suited to low nitro fuels.
This is the option I am going for on my freestyle, over the ys .63.
Nitro is many $$$$ here too, and I can get OS at wholesale prices.
The ultimate 70 is a bored out and pumped 52 and only weighs slightly more (10g or so) than the ys .63.
Its a similar priced beast though, but spend the money once and do it properly.
With the os or ys you can mount the tank on the C.G.

I know the claimed PS ratings just dont stack up- OS 70 ultimate 1.2 hp, YS .63 1.5hp, but the proof is in the prop turning figures.

The old os .70 seems to be only slightly down on power against the ys, with the new ultimate claimed to produce more power than the original .70

It depends what nitro content fuel you intend on running.

I will report back my experiences ( and RPM figures) with the new os .70 ultimate, when I get my hands on one in a couple of months.

Cameron McDonald