Forgot to mention,your local Hein Gericke store does suitable batteries for around £30.I fitted one to my 916 and it was still going strong 18 months later.
Non of you boys have a pal in the motor trade, someone with a trade account at your local motor factor? (usually no hassle warranty) Exide manufacture a tidy 12 volt 16 amp battery #EB16AL-A2, turns my 888 over with mucho-gusto and was only twenty five quid!