Hi All advise given is wise and should be noted, also consider lightening the inside of any desk/work place so as to give good reflection of light. My workbench is covered in white lino tile, a pale green as per the bergeon mats etc. as Marc says the roll top is probably the best option.
Thanks for the encouragement. I think it has to be a right-hand thread on the arbor, due to the direction the spring is acting. The mainspring did indeed have quite a bit of power left in it. There was a buzzing and a whirring for quite a while after I pulled out the balance, so it seems I have been very lucky indeed! I will check everything carefully again for damage. I have already found one cracked jewel.
That is (almost) identical to the part I have. Could not have wished for better support, thank you! I do not have the two holes in the barrel arbour, so I can't use a compass or similar to unscrew it. I will try gripping the circumference with something. The action of the spring suggests it is a conventional right-hand thread, correct?
Part of the stop-function (the disc with the finger on it) was missing when I opened the case, so it may well have been broken by another bodger who got there before me. I have the Maltese cross part. I may have a go at making a replacement if it's not too difficult. Would you have a picture of the part I need? How is it held in place?