ok, a few checks you can do to see if there is a problem.
Firstly, take out all the micro-filters, ADSL should not need to use them anymore and they are just another potential bottle neck in the system. If that doesn't improve the speed then move onto the next one. If the modem cannot connect at all without the filter then you may have too much noise on your line causing you problems (see next test).
This is a highly technical one used by many telephone engineers to test the house line. Pick up a cabled phone (no cordless) and press a single digit. The dial tone should disappear and you will have silence ont he end of the line. Listen to the silence. Is it really silent or is there a crackle, tick, audible hiss or any other noise on the line? Anything other than silence will affect your line speed.
The other one is to do a speed test, then make a phone call and do another speed test whilst on the line, does the speed vary by more than a few kbps? Try this one with and without the filter, see if the phone call is affected by noise when there is no filter.
A sudden drop in speed is often an indication of a line fault or that your kind ISP have decided you need to have your connection throttled. If any of the above show a problem then it is a line problem which your ISP needs to deal with. Unfortunately getting them to deal with it can be difficult. One of the problems with the "up to Xmbps" contracts is that as long as you have a connection they don't actually have to do anything. This should be changing soon due to ofcom pressure over the way speeds are advertised, but currently your contract is in the most basic sense "We will give you an internet connection, we will cap any speed for that at Xmbps but we won't really worry about it if you get nowhere near that. Only annoy us if you cannot connect at all as otherwise we don't care". This is a whole other rant I shall not get into here

If you value your speed then consider when you use the internet before switching over to Virgin. VM can offer speeds a lot higher than an ADSL line, but they routinely throttle those connections at specific times of the day (do a search on google about it, there is lots of information). This was put into place when the NTL Broadband systems were built and they will not turn off the timed throttle (not that they actually can).
'99 3.2V6 Auto Limited.
Some day it will be how I want it to be...