You can always do a split boil, use a standard stock pot (10L) to boil this then combine together either in a nochill cube or in the fermentor once chilled. As long as the SG of the main boil isn't too high hop utilisation should be ok.
I was wondering if I could do that actually! I'll see how I go with efficiency and if I need to will put the excess wort in a pot on the stove.
Thanks guys for all your help. Once this heat dies down I'll get in the shed and tackle it!
Will let you know how I go.