My old Rega Planar 2 with upgraded acrylic platter sounds best to my ears with record direct on the platter, no mat at all but....ymmv.
flashcider, the goal is to keep the record from resonating and slipping on the platter. A record weight will stop the record from slipping but the problem with a hard platter surface is that it has a much different mechanical impedance than the record thus it will not keep the record from resonating. You want a mat that has a similar mechanical impedance to vinyl then it will serve to dampen any resonance in the vinyl. The Funk Firm Achromat get high marks for this and I would absolutely consider getting one of these. People love them. If my turntable did not come with an excellent mat I would definitely consider getting one!.
While I do not believe personally you really need a mat with the acrylic platter the 5mm Achromat is a great choice, had one and used it for years and yes I ordered from a dealer in England too.
Just remember though you are now adding 5mm to the height your record is sitting at relative to the stylus so you will need to adjust vta to suit