Speed has dropped drastically following BT line repair

[#1]

When my landline phone suddenly became extremely noisy a few days ago, I reported it to BT and had a visit from an engineer.  Apparently there was a problem with a cable (I'm not familiar with phone technology, so I probably have this all wrong) somewhere around the block from me.  "But I've switched you to the main cable" the chap said (after several hours work) "and you should be fine now".  "The main cable" is my half-remembered recollection of what he actually said: it's close, but not quite right.
He warned me that my internet speed might take some little time to stabilise and get back to normal, and that I would see some extreme fluctuations first.  Today, three days later, this hasn't yet happened: speeds have been and still are consistently well down.  The speed checker at http://www.speedtest.bbmax.co.uk/ reports 893 kbps download, 343 kbps up.  Before the problem, I was averaging around 3Mbps download speed.
The engineer said that if things didn't return to normal I should contact my ISP, "and they'll physically go in there and do something about it".  But I don't think he mentioned a timescale.  So how long should I wait before deciding that my old speed doesn't seem likely to return on its own?
Many thanks.

[#2]

Just to make you feel totally happy and contented, my line pair was changed at the end of September last year when the voice connection went down. It took until December to fix it, and that involved seven "engineer" visits and eventually a whole fresh new line!  
Sounds similar to yours, the new line was showing a 90% throughput drop (although sync speed was the same as it had been). The guy who eventually fixed it reckoned that the first replacement line was picking up volts from somewhere.

[#3]

Nozzer, thanks for that.  Maybe I should just be grateful that I still have a connection at all.
The thing that did stick in my mind was the engineer's assurance that if the broadband speed didn't pick up, "Your ISP will physically get in there and do something about it".  I have a vision of the super-techs of PlusNet descending en masse on my little town, screwdrivers and pliers in hand...

[#4]

Hi BertCoules,
I'd pedal down on my pushbike but I'll check it out from this end first.
Jojo

[#5]

Many thanks, Jojo.
(Just chain the bike to my front fence, it'll be perfectly safe.)

[#6]

Hi BertCoules,
Hehe! Seriously, your sync rate is currently 1216k which is very poor for your line length. Are you using an extension cable/socket?
Jojo

[#7]

Quote from: Joanne

Are you using an extension cable/socket?

No, I'm not.  And apart from whatever the BT engineer did outside (he didn't come into the house) nothing has changed in my setup at all.
Bert

[#8]

Hi Bert,
Can you plug your filter directly into the test socket so I can run some more checks?
Jojo

[#9]

Jojo,  Yes, I think I can, but it'll take a few minutes.  I'll post here when it's done.
Bert

[#10]

Great, I'll hang around for you.

[#11]

Jojo,
OK, that's done.  If it matters, the filter is a double one, with both my router and my phone plugged into it.
I just ran the online speedtest again and got a result very similar to the earlier one: 919/364.
Bert

[#12]

Hi Bert,
You are definitely in the test socket behind the faceplate? If so then can you try a different filter  and if possible a different RJ11(the wire between the router and the filter)
Jojo

[#13]

Quote from: Joanne

Hi Bert, You are definitely in the test socket behind the faceplate?

Yes, definitely.

Quote

If so then can you try a different filter...

OK, different filter now in place.  I don't believe I have a substitute RJ11 cable, unfortunately.  My online speed test is now reporting 915/335.
Bert

[#14]

HI Bert,
That's still not made any difference. It would really help if you could borrow another router with an RJ11 cable.
Jojo

[#15]

Sorry, duplicate post.