I like the idea of cannibalising the sierra tank to fit onto the RH one, the cheapest one I can see on ebay is starting at £60...so I'll try rivnuts first and see how it goes, it's a tool I don't have currently so an investment if nothing else.
If it still leaks I'll go down the sierra tank route or try and fabricate something like Jez.
Many thanks for the suggestions gents