I have a 1998 car (AJ26) and some years ago fitted the aluminium thermostat tower. Whilst investigating a slight leak from the thermostat housing, I realise that the thermostat sealing ring is not under compression when fitted, as the recess that the thermostat fits into is deeper than the width of the sealing ring. I bought another thermostat to see if that was better, but same deal. I assume that sealing ring is supposed to be under compression?

 

Jaguar did offer the seal itself if that is all you need. (NCA2246BA)
You might need a different (new?) housing if the recess is not machined correctly.

I would try a thin spacer or additional 'O'ring behind the thermostat/seal assy to see if moving it forward slightly will help if that is the problem.

RTV can be used sparingly as a last resort.

 

A follow up on this. I bought a Jaguar thermostat seal and, as you can see below, it's somewhat thicker than the one that came with the thermostat from a motor factor. The moral of the story is that the correct thermostat, in terms of size and opening temp, may not come with the correct gasket!