Thank you for that, Chico. I shall look into that. As it's wireless I won't be bothered with trailing cables. The only reason I need one is because I want to use the alt key, otherwise I have great difficulty getting my French letters to fit properly ;).
The make you mention is not listed on Amazon but there are plenty of alternatives which I presume work the same way. I never knew such devices existed. You've just saved me £330.
Just to thank you for your advice, you've saved me a fortune. Together with my Amazon gift voucher it cost me the princely sum of £3.89. The only disappointing feature was that it came without a battery so I had to wait until today when my shopper brought some (I'm not very mobile and my days of nipping down to the corner shop are long gone). It works fine. The interesting thing I have noticed is that it won't type numbers if the Num Lock key is off but alt + code works whether on or off.