As per ian72 post.

Things like this or this were what I was suggesting.

Fair enough point. I think I have a BT to RJ11 lead somewhere but it was easier to put my hand on an old filter (I haven't used one since I started the FTTC trial in 2009).

Does your Huawei HG612 have a black PSU with it? If so these are known to go faulty but as your modem is out of warranty then best to buy your own VDSL modem/router.

Somewhat curious at Uno suggesting a possible £220 bill if Openreach prove the fault to your equipment ....

The usual figure touted around £130 ..... Why so much more ??

In the OP's opening post he states he was an early adopter of FTTC, so he will be using an engineer installed faceplate filter already, this should negate the having to test from the test socket bit except maybe to rule out the filter.

Me, I bet the modem is on its way out.

In reply to a post by Zarjaz:

Somewhat curious at Uno suggesting a possible £220 bill if Openreach prove the fault to your equipment ....

The usual figure touted around £130 ..... Why so much more ??

Broadband Engineer Related Charges: https://www.uno.net.uk/price-list/

Perhaps because it's for a Special Fault Investigation (SFI) visit?
They said the base call out charge is £182+VAT
Any work done to internal wiring: £44+VAT
Any work done to internal equipment: £22+VAT

I have no intention of being charged because of a fault my end, but I worry that I'll eliminate everything and end up in the worst situation; an external cause. In which case, I get charged but the problem isn't fixed and maybe never gets fixed if we can't find the source of the problem!

It does have a black PSU and I've ordered a 'BT refurbished' modem from My Memory with so we'll see what that brings.

That said, the connection has been more stable over the past 24hrs but at less than half my normal speed.

Start with the replacement modem.

Because you already have an SSFP fitted by the installing engineer that negates the possibility of the
Internal wiring modules having to be checked/charged for.

In fact the CP has various options over which modules of the SFI visit they wish the engineer to check. For instance, if they don't state the 'end users equipment' module then the SFI cannot claim to have tested that bit, and Openreach can't Bill the CP for it either.