Update, things have changed and this has turned out to be serious

Well, only kidding, not extremely serious

A week has passed, I've been off the bike only 2 days, continued working and cycling to work. A bit sore, but seemed to get better.
Was off yesterday, went for a couple of gentle walk to gather blackberries

then to the shops.
Did lots of stretching ....
Today I rode to work, back pain quickly increased, on dismounting couldn't move, had to abandon work, take the train back pushing the bike home.
Usual story on phoning the Doc, no appointments, blah blah, so I googled up a sports injury physio, walked to him

Got a torn muscle. Gave me half an hour of electro whatsitsname, some other poking too, then sent me home with an ice pack, saying I should be ok for Saturday's ride with the girls.
Heat and stretching=very bad, gentle exercise and cold over the affected area=very good.
Moral of the story: don't listen to colleagues health advice, bypass your GP if you have a sports injury (last time something similar happened to my knee, I was in pain for years) and, last but not least, don't ride a bike like you're 18 when you're 51