It will thank you for it.
These plants improve enormously from being cut down to about 6 or 8" from the ground every year, late winter or early spring.
What part of the country are you in, i can be more specific if i knew.

If you cut it down by mid February next year, you will lose the early flowers, but should get the later, smaller ones in September. Recommend you just do it this once, to allow the plant to regenerate, if you want to get the early large flowers again in following years.

Also, after cutting it back put a large flowerpot, upside down over it. This will stop the 'fencer' from trampling on it! I do this a lot when 'non gardeners' are doing work near precious stuff. No amount of saying 'mind that plant' works as well!!

Nelly Moser isn't normally cut back in spring as she flowers on last year's growth.

Thanks everyone for your advice. Am in Aylesbury in North Buckinghamshire. The fence needs to come down fairly soon before it falls down so am going to have to cut it back in the next few weeks. At least I can do it with a bit more confidence now, & I'll try the trick with the flowerpot, thanks.