Thanks Ramius, sounds like good advice.
In the meantime, I think I've located the feature on my router, under "filtering" it does allow up to 5 ports to be blocked and I've now done this.
Hopefully, it actually ought to offer you the ability to do it just the other way around. There should be a place where you can DENY all traffic by default, but have a list of allowed ports/protocols/hosts that ARE allowed to contact your network. Perhaps?