Here's a tutorial for replacing a time switch with an X10 module:
http://www.diyha.co.uk/tutorials/ad10.html
But, is your immersion heater fed separately from the consumer unit? If so, what's the rating of the fuse/MCB in there? It's that fuse/MCB that protects the circuit so you shouln't be installing anything that has a lower rating than that (cable, terminals, etc.).
If you're immersion is protected by a 16A fuse/MCB you'll probably need to put in a 16A relay driven from your relay bank. This new "relay" could be a contactor as they can usually handle higher currents. Look at part no. 729-903 from
http://rswww.com as an example.
Essentially you wire one of your relay outputs to the coil of the contactor and then the immersion heater supply through the contactor contacts. Energising your relay then energises the contactor coil, which in turn pulls-in the last set of contacts and powers the immersion.