Final update! (promise)!
Managed to buy last festoon bulb from local car spares shop around corner & now have 3 fully functional interior lighting...2 map reading lights for passenger and driver and middle dome light that illuminates for 15 secs upon entering/exiting car.
When buying bulb, did ask about LED replacements, but they don't deal in LED's! However, more than happy at moment that this is 1 less task on current list of life's 'trials'!
On separate post by zzaj, as I admit, not a electrician, what difference would a 24v system in car make as oppose to current (no pun intended) 12v system? Is it just less chance of battery drain & more powerful electricity system!