Sudden drop in speed

Having just returned from a week away on holiday I find that my download speed has dropped from 1600Kbps to 400 Kbps. Used the BT Test which says my line profile is 500Kbps and therefore nothing wrong. My line attenuation (up/down) is 28/48 and my SNR margin (up/down) is 19/22.5
Is there a simple explanation?

And the sync speed is?

Tried rebooting the modem?

Tried rebooting to no improvement in speed but after about a week the speed rose to around 1Mbps and over the last two days it has gone up to 1.8Mbps which is very good for the dodgy line which I have to the exchange which has all sorts of odd sections of cable spliced to make up the distance.

After restarting your router even though you have a higher connection speed BT's systems can wait 'up to' 5 days before increasing your IP Profile to reflect the higher sync.

NB. Your line at 48db attenuation is shorter (or better quality) than mine which is 51db and I'm sync'd at 5.8Mbs - so if you can get your line stable (internal wiring/ringwire/router choice) you should be able to more than double your speed