Put new fan belt on last night - second attempt after going back to the shop and this time getting one the right length (their mistake, not mine)
Started engine, horrendous noise, when I checked it had thrown the belt off. Refitted and treble checked it was on OK (it was dark by now) - started engine and it threw it off again.
Checked again and spotted that the pulleys aren't all in line. The power steering pump pulley is about 1/2" further forward than the others........
Next job is to find out what's going on there!!