I have just got a new battery too whilst doing my upgrades
If you look on ebay for an odyssey pc680 thats what i ended up getting
They are a little pricey but are tiny and light even in comparison to the smallest normal car battery as Spud has suggested
Although small they have more than enough juice for a roadster and can be laid flat, on its side, on its end, pretty much where you like except upside down
Got mine for £75 delivered brand new
So if any of the local builders (which there are a fair few of us in the north east
) want an ordinary battery (cvh size) i have a bosch one that i bought new last year when i finished my build, its still got 4 years warranty on it
Couple of beer tokens would secure a new home for it, obviously would need picking up though