I'm sure you will get a reminder. I think the stories about them not being issued may often emanate from people who failed to renew. But if a 4-5 days before your renewal date you haven't received one , just ring up
I got a reminder - bring your valid certificate to the post office - they will see to you.
My last 4 tax renewals were only £20. I think it is to do with the emission from the car so don't know regarding the direct debit - again post office will advise.
Thanks all - I just found the right web page, and it tells me that once you've set up a Direct Debit, that continues until the car is sold or declared SORN.https://www.gov.uk/vehicle-tax-direct-debit/setting-up
Still answering my own question - you also have to have a valid MOT and there has to be enough money in your bank account to cover the Direct Debit. Here's a better link. https://www.gov.uk/vehicle-tax-direct-debit/setting-up