Thanks for the replies!
Bought a did X ring chain and sprocket set off eBay for £55, not a bad deal. Then bought a 39t rear sprocket.
Fitting was easy with the right tools, front sprocket but was off in seconds with an impact driver and shortening the chain wasn't too difficult with a Dremel and chain breaker (took 3 links off)
39t makes first gear useable as it's not over in a matter of seconds and feels more like a "big bike" first gear. 5th has essentially become 6th with 6th only really being a motorway gear. Acceleration is pretty poor in 6th, so I'm just using that once I'm upto speed.
I think some of it maybe wind resistance so I'm fitting a screen to help a little with that.
I was looking to cruise at 70 @ around 8000rpm, I can't remember exactly but it seems to be around that mark. Speedo is under reading by around 8% now, which is fine but I've got to remember that!
I think 15 front and 39 back might be pushing it, I'd imagine 6th would become pretty useless as there isn't the power there to pull that low ratio...