We're not entirely sure which one our master socket is.
There is a socket in the cupboard where the phone line comes into our flat.
(Which doesn't strike me as particularly professional looking - there's a lot of gaffer tape, but it was definitely put in by BT, as we did not have a landline when we moved in and we had two or three engineer visits from BT until they got it to work.)
But there's nothing to plug in - when I open the little slot, there's just wires.
So we tried it from the socket nearest to that.
And: Thank you.
When I report the fault, what should I be reporting - the crackle?
yes you need to report the noise/crackle on your line when you report the fault - no mention of your broadband problem. once noise fixed that will help your broadband connection
Apparently there's nothing wrong with our line.
Well, there's something wrong with something.
Update: I followed your advice and reported the crackling again and was put through to a very friendly lady named Caroline in the Irish call centre. The check on the line came back clear again, but she arranged for an engineer to visit, who came today.
He checked everything thoroughly and also sorted out our master socket, which didn't have a faceplate on it.
Anyway, to cut a long story short, the problem had been caused by the fact that our sky box didn't have an ADSL filter. *facepalm*
Our broadband speed is back up to 2.1 - not mega fast, but a lot better to than before.