After this morning's stretches I decided to see what timer apps I could find for my Y5, that were free but without a wave of adverts either, and would be good for the exercises. I settled on two (and liked a third
The first was egg timer by Tim Hutt (see main link). Dead simple. You have a bunch of presets. Press one of them and it starts a time, with big buttons to stop or reset. What makes it good is that every one of the presets can be changed to whatever you want (in seconds, minutes, hours or days).
The second was Multi Timer "Kitty" by Christian Kaiser (see https://play.google.com/store/apps/details?id=com.chk.kitty&hl=en) which seems the opposite of the first. It's a kitchen timer and you can set up any number of timers with titles and cute icons You can re-use and edit old timers. You can also have a "pre-alarm alarm" which sounds a certain time before the alarm goes off, and you can have the phone talk to you about what's happening! It's described as a "Kitchen Timer" and I can just imagine a whole bunch of timers, each for a different task, and which can can have running concurrently with other timers. Of course that's a lot of overkill for simple exercises, but might come in handy for other stuff.
The third was the Tea Time - Kitchen Timer by Andreas Miklautsch. (see https://play.google.com/store/apps/details?id=com.andreasmiklautsch.teatime&hl=en) designed for making real tea (not tea bags)!! Looks sort of interesting.