Need a socket and spur on the same panel

Assuming this is a ring final circuit, you should use a sfcu if you want to add anything more than a socket as a spur. SFCU should use a 13a fuse in this case. A 45a cooker switch 13a socket combo accessory does not have an integral fuse. You should also consider the power draw from you electric heater and other demands on this circuit.

If you change the existing socket backbox for a dual backbox this will allow you to have a fused spur and standard single socket. Please make sure you take the current and future loading into account and remember unless your qualified you shouldn’t be carrying out the work as testing must be part of the process

I'm guessing that you have a double socket not a dual. If there is a dual back box it will hold two single outlets, so a fused outlet will fit with either a blanking plate or another socket on the 2nd hole. Using a cooker switch will fit but is not correct as it has no fuse or cable outlet. If the heater can be connected by a 13A plug then a double socket would be better. If the heater is 16A then it needs to be connected via a fused outlet, you will either have to modify the exisitng back box or replace it. This assumes the exisitng circuit is capable of taking the extra load. If your not sure it would be best to get an electrician to take a look