Mmmm... the roof may well reduce any benefit of your high gain antenna.
Loads of info here:
http://www.bbc.co.uk/reception/info/index.shtml
Check out
Television Aerials - lots of good, basic advice.
It looks like you're right on the edge of DTV reception from Sandy Heath so you'll need all the help you can get. You may be able to get more gain if you change to a band-E aerial but this won't receive the analogue signals well. After switchover you'll need a new aerial but, hopefully, you can use a local relay so won't need as much gain.
It could be that your existing aerial already points at this relay and not Sandy Heath. Look at the plastic plug on the end of your existing aerial. The colour will tell you what band it is. If it's black (band-W) or brown (band-E) then it may just need realignment.