Too much plant and not enough pot.
Given that you have clear space a few inches away, I would be inclined to cut and prepare little bed out of the lawn and plant the rose in the ground. Add some spring bulbs and some summer flowering annuals.

could be due to sap sucking critters like aphids as the bud was forming or fluctuations in water.

Many thanks both of you. Urbanite if we were fit enough to dig up the grass and all the rubble underneath perhaps I would follow your advice! Might be able to plant it elsewhere though. Seaburngirl I think on looking up close there is definitely some disease on it but the leaves etc are so healthy. I am never good with roses but this was a Ruby Wedding pressie 2 years ago so don't want to lose it!

Removing some of the worst stems now, for cut flowers indoors, might help the remaining buds.
Unfortunately, longer term the bush needs some serious pruning to encourage new growth and potting on to a much bigger pot (or putting in the ground). I'm assuming that it is Ruby Anniversary - which is sold as a patio rose but they still need good sized pots. Making the effort for it this winter will serve it much better and give you the rewards.
Take a look at the RHS webpage on container roses https://www.rhs.org.uk/advice/profile?PID=278.

You are right that it is Ruby Anniversary and thanks for your comments. I will definitely try and get someone to get it into the garden for me (will have to make room for it). Since I sprayed it many of the flowers have opened normally and it looks a lot better.