Hi Paul,
Firstly, congrats on the app!
With regards to investments and valuations, many different methods, but based the fact you have a working product, active users, generating revenue and you guys are proven in the gaming space (I checked out the wildbunny URL) I wouldn't value the opportunity at less than £100k.
Of course I don't have your full story, but if you are working on a bigger vision or there is a cool platform/enterprise strategy then you could double or triple the valuation.
Paul, I can see your UK based - have you looked in to SEED EIS? This is a very attractive, tax friendly investment - meaning you and your potential investors get more bang for your buck.
Check it out:
http://www.hmrc.gov.uk/seedeis/Feel free to PM me with any further questions
Thanks
Nick