Visitied my local garage on Friday.
They agreeed that servo assistance has failed.
Following the servo vacuum pipe from the manifold reveals that the one-way valve is located exactly where battery acid spray wrecked my vacuum pipes. Hopefully replacing this pipe and valve will fix the problem.
I also fitted a new battery at a cost of £48. Perhaps this one will contain its acid. Did a few voltage checks with the engine idling. About 13.8 volts under normal load dropping to 13.6v with air-con and others loads running. Seems ok.