I'd agree with Caterham Zetec - buy a standard car battery for a fraction of the cost - sorted permanently

To all you zetec engined cars the slow crank speed may be due to starter motor shaft length, I had similar problem, made up a spacer to move the starter away from bellhousing IIRC it was 3.5mm thick [2 Standard washers] will do to try before you bother making up a plate, this dodge has worked for me and several other KC owners I know with Zetec engines, who suffered from the dreaded slow cranking.