Problem with toilet flush valve

A few days ago I replaced the old, leaking ball valves/floats on my 2 toilets with new fluidmaster ones. The new ones work great, but seem to have highlighted another problem: after flushing, the toilets are prone to running. With these new, sharper, quicker ballvalves installed, the cisterns will then refill at regular intervals.
Obviously, it points to the siphon not sealing up correctly after the flush. I've had a good mess around with the flap open to check for any debris that may be preventing it from closing properly (I did think that maybe some debris may have got in there after flushing the ball valves through on installation). Thing is, it does stop running sometimes, so it's as if there is a point where a proper seal is created, but achieving this can take several flushes or a bit of messing around with the cistern lid off.
Both toilets are close-coupled and I know how to take the cistern off etc and how to replace the siphons. But if there is an easier solution I'd be keen on doing that. Both toilets have exactly the same type of siphon (it's one with a built in overflow) and I've taken a couple of pics (if anyone recognises the make/model, that'd be good, as I never installed these).
Finally, assuming I need to replace the siphons, where can I get new ones of this (or similar) kind? As you can see from the photos, it's a push-down button cistern, but after a quick look in a couple of DIY places I could only see ones that operate with a chain/lever.
Thanks in advance for any advice...

picture1

picture2

Dazza.

*** EDIT *** I think the flush valve might be a Wisa 2016P, from looking at this page: http://www.lunns.net/cisternspares/

Have you adjusted the float height? ie the point at which the valve shuts off the water.

I'm assuming you get a constant run over because of this adjustment, so the flush valve (not a syphon btw) should be OK.

Thanks for the reply. Yep, it probably is a bit different than it was before - as long as the water level was below the overflow I wasn't too fussy. I was actually going to experiment with it at different heights -- maybe lower the float to the point where the minimum level of water comes in. I've heard that the increased water pressure brought about by a new float valve (or one at a different level) can cause the flush valve to open a little (sorry, I kept saying syphon -- too many plumbing terms pinging around my head just now). I suspect one of the toilets does have a knackered flush valve, but the other can hopefully be saved by adjusting the float...

Dazza.

Hi there, I know this is a very old message, but I just wondered if you solved the problem without replacing that part you pictured. We have the exact same problem and have been into various plumbing shops who have looked at us like we were from another planet when we asked if they stocked that part. I have since seen in on the Lunns site, but at nearly £50 odd, I can't help thinking we might as well get a new loo!