Dave, while what you say is generally technically true, I think OP only wants to use one receiver at a time, in which case, an A/B switch should do the job:
http://www.satcure.co.uk/accs/page1b.htm (scroll down about half way to the item named 38013 A/B manual switch)
It's just like unplugging the LNB from one receiver and plugging it into the other. You will need two coaxial patch leads to go from the switch to both receivers though and then you just press the appropriate button (A or B) on the box depending on which one you want to use. An automatic one does exist (on the same page) but one receiver must be master and the other a slave. Trouble with this is that The Sky box doesn't switch LNB supply off in standby so it can't be master, although I don't know about the Freesat boxes.
However, if OP hasn't bought freesat box yet, then a tidier option might be to buy the Grundig Freesat HD box as it has an LNB loopthrough output. This means that you connect your dish to the Grundig and then use a short lead from the Grundig to the Sky box. When you want to watch one, switch off the other and away you go. Here's a photo showing what it would look like:
http://bp0.blogger.com/_IE4Jg2oU7qs/SBn40hwLY5I/AAAAAAAAAGI/9Ye5uNDwqoM/s1600-h/FREESAT+MASTER+SKY+SLAVE.jpgAFAIK
REMEMBER - all the above options are only if you want to watch one receiver at a time - not both at the same time. If you want that, then you'll need a new LNB with 2 or more outputs and another cable.