its in the opposite manifold to the right,to the rear and its in your photo,often its a 24mm ring spanner job but in yours room is available for a socket from a car socket set.
if your thinking of keeping the boiler,it may be a good idea to buy some spares now or use http://www.adlink.co.uk/ for recon spares,they wont give advice thou.
With periodic servicing your boiler will last 10 years + even more,a lot more.
ah i see it, good avice ola&bold what spares should i be looking to buy?