At just under 17stone, will a Bullet 2.5 be sufficient to have power for me as I am thinking of getting one of these.

It depends on what you want to do with your kite. If you just want to have a bit of a fly around then the 2.5 would be ok. But at 17 stone, if you want to get a real work out and get yourself in the air, you need to go a fair bit bigger. It also depends on your skill level. I suggest you try before you buy. I think after a couple of outings with a 2.5, you would be wishing you bought a biggere kite. My first real kite was a 4.9 Blade. Fantastic kite! But I had been flying flexi super 10's for a while, so I had an idea what flying power kites was all about. But first time out with the 4.9 still took me by surprise! I am 12 stone and this kite in 15mph lifts me 6ft plus. You are a big chap, but a kite like this could still get you into trouble if you are not used to flying this sort of thing. Take a kite like this out in chuff all wind for the first time, so there are no surprises! See if you can have a go on some kites around the 2.5 mark to get a feel for things, but I think you should be looking at going bigger. Read up on the wind window and where in the wind to launch and fly your kite. This is important with any power kite. Blades rule!

For what its worth....
I've just bought a 2.5 bullet. I've been kiting for just a couple of months, and for the past 2-3 times, have been out in 30mph+ winds.
At that speed, I find it difficult to believe anyone could hold the kite in position when dived into the PZ. The pull is AWESOME.