I'd  check the board  power connections.

These lamps are just hanging off the 15V supply (with individual 68ohm dropping resistors). Might be just a bad connection. 

Thanks Joe, I’ll do some more checks, just a thought are the lamps in series as When I looked one of the resistors had some black marks on the board and I wonder if this might be the cause. Thanks Dave

Hi Joe thank you just a thought are the lamps in series as I noticed some burning on one of the resistors closest to the power connections thanks Dave

The 3 lamps and their resistor are in parallel. 

Hopefully the attached snippet of the schematic shows this. 

Thanks Joe as all lamps appear intact think I will look at the transistor TR31 and see if I have 15v

thanks Dave

Dave, have you checked the 68ohm resistors? Possibly they are open or now failing high resulting in very little current flow (not enough current to illuminate the lamps)?

If the 15V supply is suspect I would think other parts of the receiver would not be functioning as well. 

Hi Joe success, measured the failed lamps and they were 90 ohms against a new lamp at 20ohms, replaced all three and now working, just need to sort the right channel with the potentiometer upgrade on the balance slider and hopefully that will fix that, thanks for your help and advice.

That's great.

I too have yet to purchase the slider repair kit.  I did a "kludge" fix using "crazy glue" on all 3 slides for mine.  So far it works.... at least well enough for both channels to pass through from tone control to power amp stage.  But I know through use this fix will fail,  however the receiver plays nicely in the meantime. 

Hi Joe just finished doing the potentiometer upgrade on the balance slider and both channels working now so can recommend purchasing the kit If you are considering it.

all the best Dave