I am working on integrating this information into UK Free TV. Note there are a number of sites with restricted radiation patterns. This information may take a couple of days to generate coverage maps for.
I have used the "Nominal ERPs" (the power levels most likely to be employed in practice by the licensee).
Lawrence Weston Road is on a slope, so what's possible/likely to be receivable may vary from property to property. However, I have made some general observations:
The transmitter which probably best serves the road is Wenvoe, the main transmitter serving Cardiff and the surrounding area, but obviously this carries Welsh regional programming.
There would appear to be quite a few trees in the way, which may produce erratic reception, this varying depending on when the leaves are on the trees, wind blowing them about, water/ice on them etc.
The Bristol Kings Weston transmitter broadcasts primarily to the south as the radiation pattern shows (go to the transmitter's page and click "Click/touch here to read Is the transmitter output the same in all directions?", which is immediately above the comments).
So these two factors -- 1. trees obstructing line-of-sight; 2. much lower signal power beamed in your direction -- may account for why you have difficulty.
rod: The most common reason for not receiving HD services is because the TV can show HD pictures but not receive them. Knowledge of make and model number would allow us to ascertain whether this is so for you.
The new HD services, carried on the COM7 and COM8 quasi-national multiplexes have an expected lifespan of five years, which runs until 2018. At that time frequencies are likely to be replanned and changed, so these HD services will probably become more widely available.