Thanks for the comments.
Battery connections and ground cable good.
I haven't changed the main relay and that may explain the momentary shutdowns but I haven't had any issues starting usually associated with relay failure, just low voltage.
I did buy an optimate solar charger/conditioner but this doesn't seem to generate enough power to maintain the battery charge.
I have a vented bonnet on the car which means the battery is located behind the duct making access to the battery a bit more awkward and certainly not something I would want to have to get to every time I use the car. Extending the positive cable with an accessible switch may be an option.
Thanks

Olly