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I believe PMS1 is top dead centre mark for the front cylinder however on an earlier post re timing marks someone said if you remove the flywheel it can be replaced "180 degrees out". I assume 360 degrees out was what was meant as there's only one keyway on the shaft. If so is there an easy way to tell or do I need to take the rocker cover off to check valves are closed?

It is the rotor of the pickup (red or black one) that can be put 180° wrong, not the flywheel of the stator

Thanks Norbert but I didn’t remove the pick up rotor - I removed and refitted the flywheel

If one day you take the pickup off and want to put it back then you should turn the flywheel on PMS1 of copmpresion (that´s when the mark on the upper cambelt pinion (the mark, not the letter!) is on 6h not visible behind the flywheel ("modern" motors) . In that moment the open part of the pickup rotor is about 4h.

If you want to change the cambelt, you have to turn the flywheel to PMS1 of "entrecruzimiento/Überschneidung". That´s when the mark of the cambelt pinion is at 12h (modern). In that moment there is no play at none of the 4 valves and you can take off the cambelt without loosing the timing (the camshaft will not turn).

Of cause there is only one position of the flywheel.

(erased the posicion of the cambelt pinion posicion for older models, because I´m not sure if they are diferent )

You can't go wrong with the position, but better check with valve covers off, the flywheels look the same (except for the first series 3 1/2), but the timing marks are on different positions.