I am looking at using expansion bottle, into which i will fill my cooling system.

I will be using a non sprung cap to seal the rad and the original over flow will be where to expansion bottle connects too.

I see i can get a Triumph Stag Bottle from Ebay which looks ideal, but any other options available??

Why not put a more modern expansion tank/filler bottle?
The setup you suggest works, but the more modern set up will help remove air and make filling easier.

What modern set up are you suggesting?

If you get an expansion bottle which has a top small inlet and a larger bottom inlet (loads out there).
The bottom larger outlet goes to a tee in the bottom hose from your rad that returns to your engine. The top smaller inlet can come from any high point on the otherside of the rad or from multiple points (I have one from the head and one from the rad on my K-series.)
What you will find is that filling is easier as the air will have a way out through the smaller pipes, you will have a constant flow of water through the expansion tank (which also removes air) and it will cause less maintenance problems.
If you want to block the original rad cap off, just buy a higher rated pressure cap than the one fitted to the expansion tank.
This will also aid filling as you can remove it to let the air out as it fills.

I'll be using the spherical one from an Audi TT. Only because I like the shape, oh and the fact that it was 1p off ebay.

I am using one from an MGF as it has a water level sensor (simple on off float switch).
I learned a hard lesson with my rear engined K-series. When you lose water, the temp gauge rises slightly until the water level drops below it, then it reads normal.
So I kept driving.
Being rear engined you don't get to see any steam either!
So after the engine repairs I wired a lamp on the dash that comes on if the level drops.
The MotaLeira is a rear BEC and I will be using the same again!

I used a Rover 200 tank from the local scrappy, see photo archive under 'Cooling System'. Also there is the X-flow pipe work diagram I used.

i used one of a mk1 polo

Thats good news thanks, as i have one from a MK2 Astra (ex Family Car) that we broke when gearbox blew (auto)

I kept certain bits, and the expansion tank was one bit, but was advised i could not use it, but now know otherwise.

Here it is, amongst the screen being trialed. I was going to mount it routhly where it sits too, by the way.
Rescued attachment 006.jpg