Redcare ~shouldn't~ interfere - at least not these days. I remember years ago (must be 10+), Redcare would interfere with DSL, faxes and PDQ machines but then BT revamped Redcare and the problem went away.

Very odd.

Just thinking in dial-up modem terms - do you have the option in your fax settings to cap the baud rate the fax connects at? It will make sending / receiving faxes slower but may make it more reliable. More of a workaroud than a solution mind.

Also thinking back to dial-up days - asking BT to increase (or possibly reduce) the gain on the line may help. Not sure if they still do this - but back in the day you could talk to faults and they would happily alter it - usually with a slightly sarcy comment along the line of "having problems with dial up speeds sir?".

I've set the baud rate as low as 9600bps but it still fails, I will see if I can borrow a 56k modem off someone and try to dialup with that.

Would be interesting if only to see what speed it manages to negotiate, if any at all.

As you say sometimes it bails out on the first or second page it is managing to negotiate sometimes at least.

It's also very odd that it can send faxes fine. Can it send faxes to the same machines that it fails to receive from?

Shot for nothing - if both your master sockets are NTE5 - try swapping the faceplates over.

Have you tried a different lead from the fax to the telephone socket? They are usually standard but some manufacturers do mess with the pinouts to frustrate us when we least need it

The very same cable from the fax works in the other socket, I have no master, it's just a double fronted thing.

I just tried sending it single page fax to a test number and it went fine, it's all very odd.

Does the light on the MCD start flashing or remain solid when the receive fails?

It remains Solid, if I plug directly into the line with a phone should I be able to hear any chirps because I can't?

Things I've tried this morning with my friends fax machine. Below applies for both sending and receiving.

Line A:
Connected directly without MCD - fail
Connected with MCD - fail
Connected through MCD and filter - fail

Line B:
Connected through MCD - no dialtone
Connected directly - success

However I successfully sent a fax to a fax to email service, and received one back whilst on Line A.

But sending/receiving to a proper machine on Line A never works, Line B works just fine.

I've gained some error codes from the fax machine.

Here's one when sending:
There is too much noise on the line for a valid connection between two fax machines to complete. The noise problem might go away over time.

Here's one when receiving:
Telephone line conditions were too poor to receive a fax.

Put there's nothing actually wrong to my ear, and faxes only use audible voice spectrum anyway?

Might be worth seeing if the BT Redcare peeps can suggest anything.

Haven't used it for ages but it should still be valid.
BT redcare helpdesk on: 0800 671 240.

Apologies if this has been mentioned.

Have you tried a different MCD ??
My brother had a similar issue at the family office and it turned out to be the MCD even though it appeared to be working ok.

Yep - could very well be the MCD going out of specification over time - usually down to poor quality capacitors breaking down.