Rain Cover???

Hope to pick up my first mk1 tintop at the weekend & need a rain cover quick.  Easy solution to nip down to Halfords & thought the "Advanced, all seasons" would do the job ok for a while at £50……….but does anyone know which size would be best for a mk1 ………small or Medium?  One of those things………too small & it won't cover everything & too big will flap about.  Any help very much appreciated.
cheers,  Paul121