Servicing the existing Siamp fill valve
might fix the problem.
Replacement diaphragm seals are available if needed. The big knurled plastic part visible in your pic should unscrew to give access without having to remove the whole valve from the cistern (but make sure the mains water supply to the cistern is turned off first!).
I'm not familiar with that Siamp valve, but if it's operation is similar to the Torbeck one then movement of the diaphragm is controlled by a pilot valve which is operated by the plastic lever. The pilot valve has a tiny (3mm? diameter) rubber seal which is subject to wear. Prising that out carefully and reversing it (or replacing it) can often cure a valve-doesn't-shut-off-properly problem.
If you do end up having to replace the whole fill valve, make sure the flow restrictor included with the new valve is fitted before installing the valve in the cistern.