The SB4100 isn't 20Meg compatible. When I had a SB4100 on XL the best I got was about 14Meg. You need a new modem anyway regardless of any faults with it.
Plus I would also complain about the fact that you're had XL broadband for 'x' months since the 20Meg upgrade without having a fully compatible modem to use the service.
(I could do an impression of Victor Meldrew but I won't, it involves moving a lot of stuff out of the way so I can get my flat cap out
) I can't believe that people still have the incorrect modem for XL, it been over a year since I got the 20Meg upgrade. It took me about a month before getting mine and that's only because I was getting problems. I wonder how many more people still haven't got the right modem for XL?