Shows 12.7V on starting, but only has energy for 2 starting attempts at normal turnover speed, then rapidly slows to stop after a further 5 goes. Voltage at end 12.4v. I'm not sure what you mean by "on starting". What's the voltage at rest, and minimum seen during cranking? Has it fired at all? I'd check all the relevant wiring and connections, including the battery terminals and engine earths: inspect, wiggle, disconnect, clean and reconnect. I'm not familiar with a Clarks Quick Starter: what is it? Do you have another battery or vehicle you can use to jump the 7? Jonathan