"Things wot I have discovered"
I bought a similar WiFi socket, the Efergy Ego, and that works great for 13A socket charging. Turns out DLink do one as well, probably based on the same base hardware/firmware (DSP-W215 I think). Both will also measure energy used but from experience the Efergy is pretty rudimentary and only does Watts and not voltage or current.
Rolec do a 16A/32A EVSE called the "Economy Boost" that has a built in timer, much like the immersion heater switch mentioned above, but GET THIS: It's not on the OLEV approved list of models - so no grant support.
Another thing - check the clock on your Economy 7 meter. Mine is two hours out (yes, I have corrected for BST!). So set your timer to compensate or try to get the clock adjusted (which is mostly hell). I set mine for 1am - 6:45am which is enough for a full charge and inside my 7 hour window based on the local meter clock.
I continue to use the Efergy WiFi plug with built in timer for now but I am going for Solar PV and ABL EVSE from Phoenix at, hopefully, end of August and I am in the process of moving energy suppliers and I have decided to move away from Economy 7 for the moment. Ecotricity, who I chose even if more expensive, allow you to change month-to-month if necessary between standard and E7 tariffs and I may move back as winters comes and the Solar PV has a shorter production window - and I will ask Phoenix to fix a manual timer.