So... update. Due to apparently some parts being incompatible between the SD200 and SD253, I reverted to the SD200 control panel and heating element but retained the SD253 motor and pulley as these allowed the use of the wider drive belt which will be less likely to stretch. I have just made a loaf and the bread is as it should be... success.
In reflection I think the problem was the drive belt and not the capacitor, although I now have both a new drive belt and capacitor.
One thing you need to be exceptionally careful about if pulling apart any of these Panasonic breadmakers is this. There is a ribbon cable consisting of 9 wires that goes from the display panel to the circuit board. If this ribbon cable pulls out from the circuit board, it can be very very hard to get back in. If the individual wires, of which the 9 wires in the ribbon cable consist, beginning to separate, it is extremely difficult to get the 9 wires back into their holes... the unwound wires bunch up and do not go though the holes, then touch each other. I know! It took me a couple of hours and I only succeeded because I had a spare ribbon cable from the SD253 to use, and even then I'm not 100% happy with the connection. The original SD200 cable was to too short to redo the individual wires. If any of the 9 wires touch each other you will get an error message and the machine won't work. The fitting on the board should be pushed to open the connection points slight to allow the wires to go in.