[Arran]

 I don't think that a brightener will help much with an electrostatic tube like a 7JP4, when those tubes get dim it's because the phosphorus has become fouled not because the cathode emissions are poor. What may help is if you could increase the high voltage somehow, did you replace the HV rectifier yet? Even when these tubes were brand new they were never as bright as a electromagnetically deflected tube, and were often viewed in a darkened room. It's too bad that this set appears to have had a high number of hours and has worn out the CRT, on most of these electrostatic sets the set was retired long before the tube became unusable, probably when the owner decided to buy a 21'' set or when the high voltage caps started to fail, or both.
Regards
Arran