I agree with the others, Soapy. A fused spur (13A fuse) is perfectly Ok and safe, but not the best way. A dedicated immersion circuit is much better.
Your socket ring will be either 30A or 32A. An immersion is likely to come on quite often. Add a kettle/electric fire/washer/dryer, and the circuit is likely to be well loaded. This will depend on how you're wired up. It's good practice to put such things as a utility room or immersion on their own circuits.
Don't use a standard 13A socket. The plug will get very warm, and you'll likely start to see scortching around the pin entries on the socket itself.
What a lot of people don't realise is that, even if a 32A ring circuit is running with a 32-35A load, it may well be vulnerable. This may be just being a bit fussy, but if the load is "off-centre" in the ring (most of the load being near the end of one leg as it enters the consumer unit), then it can push that end of the leg towards overload.
Let's not lose sleep over this, but it does explain why dedicated circuits are preferable.