Cheers for comments guys much appreciated, I have considered the non ryobi batteries but heard bad reviews also but maybe worth a punt considering it's like a 3rd of the price!! The bigger the batterey the more power it generates for them harder/thicker jobs from what I've seen anyway Thanks again guys
Unless you are a pro and need to use the tool all day you wont need a battery that size. I've got a 1.5ah for my Bosch and it lasts a very long time just doing diy jobs.
Amazon have a non genuine 2.5mah battery in the warehouse for £10.77, like new. I have a 5.0mah which looks the same, it seems as good as the original, lasts ages. I was going to buy the 2.5mah but the reviews don't seem great, I'm not sure if I need another battery really. Search on amazon for B07QPTNG9P and you'll see it.
A couple of years ago I bought some non genuine 4ah batteries (the max available then) from ebay for approx £20 each - they've been great, don't think I've ever have them run out on me. Looking now looks like you can get 2x 4ah batteries for £35 whereas the 5ah are nearer to £50 for 2. I'd go for something like these.... https://www.ebay.co.uk/itm/2X-4-0Ah-18V-Lithium-Battery-for-Ryobi-One-Plus-RB18L50-P102-P108-RB18L40-P104/264450941534?hash=item3d9281225e:g:6OkAAOSwprte6cKP
Amazon have offers. Add Ryobi to the alert section here so you don't miss when they come up.