OK, the speed drop coincides with the home fuse box problems, the Openreach router was reseting when the fuse box was tripped or being disconnected when we had building work done over the last few months.
Will the DLM automatically improve them speed if the connection doesn't disconnect?
If your modem was resetting that could cause DLM to kick in. If the disconnections have stopped DLM should start to raise your speeds again. This is an automated process and can take up to two or three weeks. Keep an eye on it and if it doesn't improve post back.