Yes the voltmeter was working afterwards - I've also used it again to check out your suggestions
With a lot of scraping off dirt I managed to measure the right voltage between the battery to chassis ground (cold and with engine running). It took a lot of tries and a lot of scraping though. For the first couple of tries it was a volt below the voltage between the terminals (hot and cold). When I scraped enough though I managed to get the correct readings. I'm wondering whether this ground might be worth cleaning up? The bits of the bolt that are still dirty and the ground cable itself still give much lower readings. Could it be that the dirt is acting as an insulator and making the ground ineffective? I did not have the same problem with the other engine grounds.
Reading between the engine block and -ve terminal of battery was - 0.02V with the engine running, is this the kind of low voltage that would indicate a bad ground ?
I tested the brake servo using your method and the pedal sank quickly - the engine was not harder to start.