Take a look at the boiler. Its type should be written on the front.

If you have a Potterton Powermax, then they're a unique type of boiler, but best fit into ~~System.~~

Here are a few general rules of thumb:

If you have a cold water tank (probably in the loft) and a hot water cylinder in an airing cupboard, it's ~~Standard~~.
If you've no cold water tank but you do have a cylinder for your hot water, it's ~~System~~. These boilers often also have a pressure gauge on the front.
If you don't have a cold water tank or a hot water cylinder, it's likely ~~Combi~~.
If the unit is built in behind a gas fireplace, then it's a ~~Back Boiler.~~

If you're still unsure, give us a ring on {{customerServicePhone}}.