I thought it would be worth mentioning that we tried to buy the Volvo version of the Multi-Tech after reading about the massive savings on it by buying through Volvo and found that it was £244 (inc VAT at the correct rate) through our local Volvo dealers. I am not sure if it is the case for all Volvo dealers, but it seems that the saving is no longer as the Britax branded one is about £250 online.
In the end we went to the In Car Safety Centre in Milton Keynes and got a couple of Britax TWE which seem much better suited to our needs, more comfortable for DD and cheaper.