Well, after trawling ebay for a month or so I dropped on this tank bag that is suited to the plastic tank as it just straps on instead of magnets. Cost me £20. Think it suits the bike.
It doesn't have a model no on it that i can see however it is the expandable humpback tank bag in the lifetime category, it came with the straps to attach. If you have a look on ebay there are strap on bases for the lifetime luggage for about £13. So if you already have a tank bag, above the first time luggage, with a detachable magnetic base you should be able to just buy one of the strap ones.
