You're a bad man Ronnie
That's why I like you.
I have found a Sikaflex equivalent, looking at the respective datasheets it has the same hardness when dry and the same strength, it's available in white, grey and black and it's a hell of a lot cheaper. It's called Polyflex. Like Sikaflex there are different grades available, they do LM and HM which are equivalents to 221 and 251 they are both high modulus moisture drying polyurethane preperations. If you search for it on the net you can find the datasheets and stockists that do it for about 3 quid for 300ml. One seller on ebay does two tubes for £4.50.
Hope that's of use to somebody.
Enoch.