Thanks for all the advice.
Having spotted the battery a couple of days ago I bought a new one yesterday and replaced it. At the time I did not know I would have to reset anything and found the same problem after replacement. Pressing F2 after powering up took me to a screen where I could change some settings. I discovered the date and time were wrong and reset these, the machine seems to start fine now (only tried a few times so far). The only issue now is that the image size seems to have to calibrate itslef to the monitor size on each start up, it didn't do this before.
Thanks again,
BB