I did my old mum's gravity HW not long ago. Put a 28mm motor valve and an extra pump on, plus a new wiring centre and changed the programmer wiring. The 2-port valve has a microswitch in it that turns the pump off. The 28mm valve has an extra wire that the 22mm valve doesn't, for that purpose
A few observations:
It speeded up the Cylinder heating time enormously, and cut down the summer gas usage by about half (the boiler is a big Potterton Profile with a lot of iron in it)
I insulated the HW circuit pipes with thick Climaflex as there are a lot of losses from those 28mm pipes in a long run (even though, with the cyl stat and 2-port valve, they are not hot all the time any more)
Pumping the HW circuit dislodged a lot of old sediment out of the pipes and cylinder (I had previously cleaned the system and added a Magnaclean, so I knew where the dirt had come from)
I chose to add the 2-port and extra pump because, due to building work after the boiler was installed, and there was an Upstairs and a Downstairs CH circuit, I was scared of having to reconfigure all the pipes in a constricted space to put in a 3-port valve. I had picked up a 28mm 2-port and a new pump at good prices (28mm valves are particularly expensive, especially the 3-port). I did it with the help of my BiL who is a refrigeration and air con engineer, and competent at pipework and wiring controls. I found the controls quite difficult to grasp and got lots of help on here.
I had fitted gate pump valves, and turned them down a lot as it tended to get slight unwanted circulation through the CH and would briefly pump over at start-up if CH was already running.
On consideration I suppose it would have been better to bite the bullet and fit a 3-port valve, though I would probably have had to pay a pro to do it