Why are you replacing the dizzy? It sounds like you need a new cap and rotor arm. Is the dizzzy worn? Do you have play in the dizzy shafts?
As for the coil, it doesn't really matter, as long as it has the right resistance rating. You can use a multimeter to check the resistance of the primary coil on your current coil, 1.5 ohm for a balast system, 3 ohm for 12 volt system.
When you wired up the britpart kit, did you use a 12 volt supply? If you take the power off the coil of balast system, the module will get 6 volts instead of 12. It will probably work but it won't be good for its longevity.
The coil in your link looks to be right for a balast system. I would be a little careful buying off ebay, some of the coils sold there are made of cheese but if you trust the seller, it shouldn't be an issue.
Hi, I was gonna replace the dizzy as it just feels like there is a bit of play, and at £100 for a new one plus plugs, cap, leads and coil I think it makes sense, especially as I need my car 3 days a week for work.
I will check the coil tonight and confirm if its a ballast or not.