Although I agree with all the replies so far, not many of them address Ian's question, which was what to say on the radio!
Ian, if CAP413 doesn't tell you what to say (or if you can't remember the exact wording), then English works fine!
Also, if you're not returning to base, but diverting to a different airfield, make sure they know it's a diversion - many airfield will waive the landing fee. I've used r/t along the lines of "G-XX is a PA28, originally from A to B but diverting to you due weather, 5 miles south at 1000' request joining information" in this situation. (Obviously request joining instructions, rather than information, if it's ATC.)
FFF
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