If the valves are old then you would be better served replacing the valves - they do wear out or become sticky, and this then compromises the control of the affected areas which defeats the purpose of adding the controls in the first place.
I don't know what connections the Conrad system uses as standard, but I can recommend the Honeywell TRV bodies as these are some of the most reliable around. They can also be serviced in-situ and have the innards replaced too should they need attention in future. The screw fitting between the actuator and valve body used by Honeywell (and now many other manufacturers) has become a sort of industry standard, so stick with this fitting if you do decide to change.
If the Conrad system uses the threaded connector (like the Honeywell HR80s) then you could potentially buy the Honeywell adapters for certain modals of TRV, however the costs are more than likely more than the cost of a new valve body (although not the cost of the plumber to fit them), and they are not commonly available, so most suppliers would need to make a special order from Honeywell.