UPDATE: I've sent the phone for warranty repairs and the issue (of course) isn't happening for them. They said it could be my router. I've got an Intellinet router. So I did some searching and someone had a similar problem with a different HTC phone
HTC Droid Incredible keeps disconnecting from Wifi
Funnily enough I have just been borrowing a mates HTC Trophy and it seems to be having a similar issue.
I also noticed the problem has been mainly happening when I'm in my bedroom.
So I went on the admin page of my router and noticed it was on Channel 11. It had a function to find the best channel. It shows that Channel 11 and Channel 3 were being used by routers near me (it also highlighted the SID using the channel). I noticed that the strength of that SID is a little stronger in my bedroom so maybe, just maybe that's whats causing the issue. THe router suggested channel 5 so I've changed the channel of the router to 5 and will now see if the problem resolves.