Thanks for the replies.
Roy- thanks for the pictures. Yes I had come across machine spares – it's good that spares are available but they are a bit pricey. Compared to the equivalent new price of the drill they are probably reasonable but when you only paid £20 at a boot sale for the whole thing, a new spindle that comes out at about £150 inc VAT makes me think twice (luckily the spindle on mine seems good).
Clive – you may well be right about the differences, I am beginning to think that there wasn't a great deal between the Mark1 and Mark 2. Pat's picture of his old Bantam suggests that there were two completely different Bantams.
As far as speed is concerned, I am minded to put a three phase motor and inverter on it which will allow some lower speeds and save a few belt changes.
I did get caught out recently, before embarking on this research – I wanted a new chuck and thought the chuck removing collar was the remains of a collared chuck so ordered up a Jacobs 34-33C (the spindle taper is JT33). Wrong! The removing collar is threaded 1" diameter and the chuck requires a 1 1/4" thread. So I have a brand new 34-33C chuck if anyone needs one…
Tim