Thanks to all for the helpful replies. It is encouraging that others with back problems have been able to manage this issue and maintain aikido practice.
I also heed the advice to be careful. Three or four years ago I couldn't have contemplated this but with the improvement I have had in the last couple of years I think it is a realistic possibility. I appreciate that a careful approach is in order and would certainly have a discussion about this with the instructor should I go ahead.
One of the dojos I am considering has most of the training sessions in the early morning (ie 6:30am!). This may be an advantage since it would minimize the amount of interference with family life (eg helping the kids with homework in the evenings) but I do wonder if it would be okay from the point of view that you tend to be a bit stiff in the morning and the back more vulnerable.
Just remember to take things really slow in the beginning, and you should be ok. Also make sure your fellow students know you have problems with your back, so they can adapt to the situation.