The Rangpur limes are the most hardy of all our citrus. They are also the first to fruit out of our young and very small citrus trees! The only other fruit we've had so far is a few lemonades.
Anyway, I saw this recipe for lime butter in a great book I have called The Country Store by Stephanie Donaldson.
Very easy to make, it's just 110g butter (more or less), the juice of a lime, a little zest and some salt and pepper. Whizz in a food processor then roll up in greaseproof paper. Slice into thick slices and use on fish. I made 2 lots which will sit in the freezer until needed.