Well you have said it yourself - diversity.
Have you thought about speaking to an IFA?
Secondly you state that you currently rent - so I would consider investing into something perhaps no longer than 5 years, as you may want to switch to getting a mortgage.
Also for example - you can get up to 6% in a regular saver - Regular savings accounts: up to 6% online - Money Saving Expert.
Would that be an ample return - so on 12 x £300 deposits you would get 108 in interest, on top of your £3600 saved(taking their top example - not sure if there is better).
Another point could be to add the 400-500 a month to your AP bankroll. That has the potential for the greatest return.