First things first, with those figures you are not in a weak signal area. You quite possibly were in the days of analogue but you should be receiving a 46dBuV/m signal from the Rowridge transmitter. Whilst it might be 35 miles away it is pumping out 200KW which is considerable (certainly compared to the analogue days).
Now, that is what the figures say but local issues such as large buildings, trees, etc can make quite a difference - as of course can the aerial type and quality of the aerial, wiring, etc. Starting with local topology grab a compass and look in the direction 125 degrees to see how far towards the horizon you could likely see at aerial height. The aerial needs to be a so-called 'Group A' aerial (different aerials receive different ranges of frequencies and these frequencies are grouped by letter) and mounted with vertical polarisation i.e. the elements should be orientated up-down (not left-right). Your transmitter providers horizontal polarisation too and whilst it might be at a slighly lower received strength at your location you may find it provides better results depending the local environment.
A decent aerial would cost around £20-£30 (
example). The recommended aerial type is chopped off your screenshot but it looks like it might be saying an amplified external aerial i.e. that you'd need either a masthead amp or internal one in the down lead. This won't necessarily be the case though if you go for one with a high number of elements (e.g. the 18 element linked to).
There is every chance that you could mount an aerial on the granny annex. I receive a weaker signal than you (still what could be regarded as 'strong' though) and get perfect results from a loft (i.e. internal) aerial without any amplification - indeed the signal is passively split to two points (it is split further but only two are ever in use).
Before you do anything try and work out where your current aerial is pointing. Do you have any means to test it? I don't remember if you said it was full wired up or not.
