http://www.ebay.co.uk/itm/UBEC-5A-8-40v ... 3429wt_907

Thats the switching BEC I use, fairly cheap and weights pretty much nothing (21g) and can run at up to 40v. There is a cheap one of the same model that works up to 26v and weighs 3g less, but if you're bot is only weighing in at 4.5kgs you can probably afford the extra 3g of weight . Saying that though, if the bot is only 4.5kgs you can probably afford the extra weight of that electronize providing the BEC.

Does the BEC not work on that one I gave you? A few people said that soldering new wires on might fix it, so might be worth a go.