[neilball]

You will need to speak to a KNX dealer for pricing and it does allow you to mix and match different keypads and dimmers from a number of manufacturers. Ivory Egg may be worth a call, I think they are trade-only suppliers of KNX kit but they do seem to be building up a wide range of installers and could give you some advice on cabling.

If you are looking to install over time then Rako could be worth considering as the wireless comms between modules and simple commissioning make this a practical DIY option as long as you can find a retailer to buy the kit from.

Another option could be Helvar if you prefer hard-wired controls - they have just intrduced a packaged 8 channel dimmer at around £1k and their keypads are around £130. It can be wired fully in twin & earth cable so pre-wiring the other rooms is very straight forward and you could also use the existing Cat5e by doubling up the conductors (it uses the Dali network protocol which supplies power and data down the same cable). Cat5e is not usually preferred but the Dali standard seems more tolerant of using non-spec cable especially in a residential environment where the cable runs are not massively long.

If you are looking for a pure DIY install then this may restrict you to Rako and Lutron GraffikEye as most other systems are only available from resellers who would want to commission/programme the system for you.