It really depends what stations you want to pick up.
If they are local and fairly strong, you may find that simply fitting a diplexer to the incoming TV aeriel lead and a pair of flyleads )one to the TV, one to the receiver) will suffice. There may be some signal loss to the TV as well though, so if you are in a weak TV signal area already...
They are only a few quid. Maplin do one for a fiver, FE27E.
B&Q will probably have the same thing too.
If you want to pick up distant/weaker stations, you'll need a proper aeriel and downlead (as well as a good tuner of course
)
You can also fit an FM aeriel and use a diplexer at both ends, so you can use the same downlead - but IMO if you have to go to the trouble of fixing a new aeriel to the chimney, you may as well fit a new downlead while you are at it - it probably won't cost much more than the pair of diplexers anyway, and you'll be certain of no degradation to the TV signal from any diplexers.
A diplexer looks just like a Y splitter (in fact it's just a modded Y splitter), but it has some components in it to "split" the VHF FM and UHF TV signals.