Everything IDN has written is correct but I'm wondering just how close to the end of 2008 it was when you bought the baby monitor? It's relevant, as I'll seek to explain:
Ignore the guarantee for the moment. A retailer is obliged to provide a repair or replacement for an item that fails due to an inherent fault for (at least) 6 years from the date of purchase. (An inherent fault is one which was present at the time of purchase, such as the use of poor quality components). For the first 6 months after purchase there's an assumption, in law, that any failure must be due to an inherent fault (unless, of course, there's clear evidence to the contrary, such as signs that the item has been dropped). Thereafter the purchaser has to show that the most likely cause of the failure was the presence of an inherent fault.
Since it's now June, you're very close to (or possibly past) the end of the 6 month period where you can simply say "It's stopped working. Fix it or replace it". If more than 6 months has elapsed since purchase you might also be required to show that an inherent fault was the most likely cause of the failure.
Guarantees are usually offered by manufacturers, rather than by retailers. They're entirely separate from your statutory rights with the retailer. Hassle Mothercare for a replacement but, if you're getting nowhere fast, remember that you can also claim against the manufacturer under the guarantee.
Chris