Thanks, having had a fiddle around it seems to be a contact in the B/Auto/Flash selector ring on the front of the lens. If I jog that around a few times it works for a while and then stops. I notice this because I put it in B and the shutter wasn't staying open but after a few turns of the ring it worked for a while.
Does the selector ring feel loose?
If you fancy having a look yourself it is not too difficult to remove the selector ring.
First you need to unscrew the front element retaining ring - either use some sort of good fitting snipe end plier or circlip plier, or better/safer still, if you have a rubber item that fits the ring well, apply pressure and turn using the friction of the rubber - it's easy to slip on this ring!!! In fact, looking at the tiny slots again just now, I would suggest some sort of rubber disc to remove it - I can't remember what I used with mine, but it couldn't have been any sort of plier as I don't have any that small!!
After that, there is another retaining ring to remove in a similar fashion, then you will see three small cross point screws, remove these and remove the front element part of the lens. Watch out for a small ball bearing, this needs to locate in a small hole in back of this piece - it's what keeps the selector ring clicked into each of the 3 positions.
Now the selector ring itself should be loose, rotate it back and forth gently whilst wriggling it to remove. Be very gentle, there are the small contact springs preventing you from removing it straight off - just lift it a little and wiggle/edge it out to ease the springs out.
Now you should see the contacts on the front of the lens - clean these up and check that the solder connections onto the ring are good - also clean the sprung contacts on the selector ring.
HTH