Success with a Dot! I phrased it differently though. What worked was 'Alexa, remind me to [feed the birds] in 3 minutes' and it did so. There's a different alert sound too. I'm just waiting for the second reminder [to feed the cats] which, on reflection, might have been better coming first
Dot's software version is 578274620
Aaaand I've had a bizarre conversation with this thing now. First asking about what was left on the timer and discovering there wasn't one (because it's a reminder, of course), then asking about the reminder and being asked what I wanted to be reminded about (suggested cats again), then the reminder going off and not stopping when told - I tried Stop, then Ok, and even Thank you (which got me 'That's what I'm here for' but it still didn't stop), then another 'STOP!' which did the trick just before the reminder I'd set because the first one hadn't apparently been saved going off and another interactive palaver commencing! I can see I'm going to have to work on what Alexa wants to hear or we're going to be having some quite exasperating conversations!