Lowest download of all time - baffled

[#1]

I've been away for three days; prior to leaving home d'loads/up were low but still averaging 2.6 meg & 0.4;  I come come back today to find Down/up show 0.44 & 0.4.  BT speed tests for both occasions were 2642/448; DSL 4512; IP 3000 and now 416/448; DSL 4960; IP 500.
Whilst away for the three days, I worked on my daughters PN set up; fitted new ADSL/Nation XTE faceplate, ran separate ADSL cable & separate voice into next room for her l'top & phone.  Result - complete satisfaction with ADSL d/load speeds doubled and steady.  Connecting/disconnecting plugs seemed to make no difference to her set up for BT/PN IP profile @ Weybridge, yet down here in Haslemere, every time I occasionally shut down modem/router or pull a plug out, the system gets back at me with lower performance. 
PN have arranged for BT appointment this Friday 29th [ticket 31200280 been open for some time - problem falling speeds, unsteady drops and complete drops in service on occasions ten days ago] but any thoughts as to why the disastrous drop off, please?
Incidentally, my Belkin modem router had not been switched off in the three days of absence, however when I set my l'top up on returning home, the above is what I experienced.

[#2]

Update as of this morning, Frid 29/1:-
- for three days, line D/speed has stayed on 0.4, the lowest I have ever experienced; miraculously shot up last evening to 3.1M and has stayed at this for last13hrs.
- BT engineer called this morning, spot on time; v. helpful & given my set up/system a clean bill of health.
- Continual drop outs which show up @ PN's end are still a mystery [are they to do with PN or the BT xchange?] knock back my D/load speeds and unable to enjoy a stable system.
- so I await PN's diagnosis and what to do.  My ticket is still open.

[#3]

Saturday evening 30/1 and second update since BT engineer arrived on Friday [he confirmed my system was all OK!].
From dropping to  0.4 Mbps downloads for three days, I am now on 4.3 Mbps down & 0.4 up; the best I have ever has since joining PN.  BT speed test shows IP profile of 5000 [this too had dropped to 500]. So something has happened somewhere, what or who has done anything I do not know - for how long it will sustain this [for me] dizzy d/speed, remains to be seen.  I mention it not to crow but just encourage others. 

[#4]

Well, I'm three days into a 'new' way of living. 
Since PN sent their BT engineer, who seemed to do nothing [I wonder!] other than confirm my set-up/equipment were OK, I've had consistent b'band and speeds of 4.3 Mbps down 0.4 up.  Never had this since joining last August.  Through the forums I can see the complexity and number of factors that can affect one's b'band but whatever was done behind the scenes has worked wonders.
It's been a long opened ticket but I'll just wait a little longer before closing it as SATISFIED.
Oh joy. Will it last is the big Q now