Hi Terry,
Yes you can use tablet fuel providing you place it in a suitable container... with a raised lip all round.
Whether it will provide more heat, than Meths, is arguable... but certainly worth trying and is easier than wick trimming.
You could also use a smalll round ceramic gas burner which will provide considerably more heat than either of the above.
You state that it is a very old unit... so may have a build up of calcium/limescale inside, which will severely restrict the transfer of heat to the water. This would be more likely if the boiler had been used with hard tapwater.
It may be worth filling the boiler with some 'Steam Iron' descaler (preferably hot) and allow to stand for a few hours... empty and rinse well... then fill with filtered rainwater or distilled water and try steaming it again.
Hope this helps.
Best regards.
AlexC.