Just put a new battery on my mk1 fabia vrs, connect battery and car turns over straight away with ignition off, jammed starter solenoid?

Sounds like what you think, does engine run, or does it just keep turning engine over.

Give starter a thump on the outside casing, was an old trick usually to get it working, might work in the reverse?? to disengage solenoid/ starter.

Yes, jammed solenoid, block of wood and hefty hammer will fix it.

Yeah agreed, oddly enough didn't think of that straight away when you PM'd. A stuck solenoid is most likely.