Thanks for your responses guys.
After following the link provided by HGR3inOK a few days ago, I did conclude that the AV7s were the correct ones, my tyres are 40-406 (20" x 1.5) so that's what I need. BTW, I still don't know what the 40-406 means - I'm guessing the rim diameter is 406mm, but the 40? Wassat?
Just out of interest, here are a few pics (not great quality) of the original Kenda tube and the AV7A's that Evan's Cycles (incorrectly) sold me - you can see that the 20" x 1 3/8 AV7A is way too big, even though it's supposedly a 20" 1 3/8 tube which to me would mean that it should be the same diameter but a wee bit smaller than the 20" x 1.5 that's correct for my bike.
I'm confused by all this tyre/tube sizing as it doesn't seem to make sense to me, even after reading Sheldon Brown's pages about tyres and tubes.
So just to confirm, I need
these, but do I need to specify the exact size (i.e. 20" x 1.5) or is that size covered by the tube that is 1.50/1.75/2.00?
Thanks again