Hi Graham,
I found it... It has an old "V640PX" battery which you can't buy any more so people use 3 stacked LR44's (I think) - but the needle deflects when the trigger is pulled and varies as you would expect given the reflectance of the target.
It's in excellent condition and has all the parts (lens cap, zipped case, strap, manual (one printed A3 sheet but the original). I'll put some 1.5V button cells in it but it seems to be fine. If you have an email I could send you some photos I got my kids to take...
Neil