Well I've just layed my Trakmat straight on to my carpet with my track just resting / laying on top & I have not used a cork underlay - now saying that - I'm not sure how you got you Trakmat but every time I bought a Train set from Hornby - most of them come with a Trackmat & Leaflet & a Owner's Manual (which is printed on an A3 sheet of paper) & it tells you about Baseboard Construction & taking a quick read of it - it doesn't say any thing about a cork underlay!!!
It say that a typical base board of 1800mm x 1200mm - It says The best meterial is an insulation board like Sundeala backed with a frame of softwood battern, 50mm x 25mm or bigger, running along & across the underside of the board at inervals of 600mm or less.
Before you fit the top, cut notches in the upperside of the internal frame members so you can route wiring under the baseboard - if/when you need it - etc
It also say if you Trakmat is made of paper use PVA Glue it stick it with or if plastic use double sides tape.
Hope this helps