The Hartz Just for Cats Mouse Toy With Bell is sold by Asda at £2 for a pack of 2. I picked a pack up last time I was there, just out of interest. They seem to be slightly unreliably made – the yellow one is fine, the red one unravelled and the bell fell off first go, but Pete loves them.
He likes them so much that he doesn’t even need me to roll them for him – if he’s in the mood for mischief, he’ll start hunting them spontaneously. He goes full on crouchy wiggle-butt pounce for them, and gets so engrossed he runs behind doors so he can hide and pounce on them from a position of stealth. He’s had them about a month and not got bored of them yet – well worth the money.