Welcome to the forum.
Presumably you are using the Sandy Heath transmitter. Firstly can you check the sig strength and quality DVB-T2 UHF 21.
You are so close to Sandy you may need a signal attenuator. If you have a splitter try fitting one to reduce the signal strength and check again. If there are any amplifiers remove them first.
Is your aerial a communal one or your own ?
Long shot wonder if your last aerial was using a different transmitter to Sandy. Check the following to see if you find a HD signal.
30 (Crystal Palace), 50 (Tacolneston), 47 (Sudbury), 58 (Waltham)