Any speaker plugged into your earphone socket should work... there are ones available which include an amplifier (for more volume) and those that don't... The ones with amplifiers are a bit heavier and bulkier...
I'm just wondering why the phone's speaker isn't working...?
I'm not 100% sure why it's stopped working, but the phone stopped giving audible notifications over about a 2 week period. I had replaced the battery about a couple of week earlier. One of my friends pointed out that the case wasn't on properly when he looked at it, and that it did look like there was a lot of pressure on some parts. I'm now guessing that I'd messed up putting the new battery in and I've physically damaged the phone.
Thanks for that. Ideally I'm looking for something small as I'm going to have it plugged in quite a lot. It doesn't need to be massively loud, as I can always plug it in to my normal speakers when I'm at home