ADSL2 +

[#1]

If they were to do that how long would it be for my speeds to return normal?

[#2]

That would normally be upto BT but usually it happens over the course of 2 days to a week (I think but don't hold me to that)

[#3]

Wrong - it's probably down to Plusnet not BT.
On 21CN (ADSL2+) BT set the IP Profile to 88.2% of sync speed immediately upon synchronisation. What tends to happen is that Plusnet are either tardy updating their copy or lose the update altogether.
Run a BT speed test and see what it says for the IP profile. The go to https://portal.plus.net/my.html?action=stable_rate and check that the current line speed is the same (rounded down to the nearest 100).

[#4]

My mistake thanks for the correction Jelv