Well done, it is always good practise to change the bulb first, particularly if you do not kow its history. The next step would be the control block and /or regulator. As most people would not want to fork out for a new one, at £540, S/H is the best option and you still get to use the new bulb.
The only unknown then left is the regulator pressure and it is rarely incorrect, so a reasonable risk. (£30 to find out)