I looked into this a fair bit - there is a lot of info on the Triumph forums. I have not followed-up on roto-flex so the text below is only relevant to Heralds and MkI and 2.0l Vitesses.
Consensus seems to be -
Fitting a rear antil roll bar on the conventional Herald spring can help a little, but you will almost certainly have to make something yourself.
Beefing up the front a/r (Spitfire MkIV or later) is the move of choice (you will need to widen the holes at the mounting point) but some people seem to think that this should be done only when fitting a swing spring from a similar car (this is an easy swap).
As Mr T says, going lower by at least an inch front and back will also help handling a lot.
Canley provide swing spring kits (link below) but you can buy the bits yourself for about a quarter of the cost.
The link below is a useful site to start with as it introduces the "camber compensator" which are not easy to come by -
As always there is a fair amount of contradictory chat out there. The link below has contributions from those that have tried rear a/r and camber compensators and most could feel very little difference with an a/r (and not much with a CC).
One interesting suggestion is to increase the rigidity of your chassis. Get that welder out!
Oh, and make sure that all your bushes are in sound condition. That's sound advice for any man.