Update:
I have been fortunate in being able to contact a gentleman who actually built and serviced these beasts.
he has written this to me:

"Thanks for your enquiry with regard to your problem the shutter was introduced to the Devere twin lamp lightsources to reduce the after glow of the lamp once the timer has switched the lamp off after a exposure. The solenoid that operates the shutter could be the problem or the power supply itself which had a separate supply for the solenoid.
Unfortunately I do not have a solenoid but maybe the best option would be to find a electrician who could tell you if the supply from the stabiliser to the solenoid is ok .

The shutter system was never that successful and I have seen quite a few 500W lightsource where the shutter has been disconnected."

he then added...

"From memory there was a small transformer that supplied power to the solenoid and I think depending how old your system is they were located in the voltage stabiliser box or in the lightsource itself but you would need to take the side casting off to get access. I think you could use the system for B/W photography with no problem without the shutter working. It was only a problem when used in conjunction with a motorised roll easel where multiple exposures where used.
Good luck with the machine."

I thought I'd post it in case any other fellow forum member who own a 504 are interested.
i will check what he mentions but if it is too much trouble to fix it I will follow his advice and just leave it open all the time.