Autoglym do a two-part cabriolet cleaner and protector kit which would be ideal for the canvas roof. Not used it myself but swear by autoglym products in general.
It does have high reviews on dooyoo (5 stars) and halfords feedback section (4.4 out of five), 4.5 stars on Amazon.
The kit is £20 in Halfords but can get it for £17.95 on amazon including delivery (the same company does the £40 HD wax for £25.95!)