Some solid advice, thank you.
I was thinking heat might possibly contributing to it in some way as it was over 30c here when the issue initially started a few weeks ago. The last couple of weeks have generally been cooler and without issue, however it really has got warmer again this week. So its certainly a common factor.
The engineer who replaced the modem last time did also fit an amplifier, similar to this:
https://emea.technetix.com/product/p...ts-unity-gain/