Very old clematis not flowering
 

I have inherited this old clematis with roots as think as a child's arm. It has not flowered last year and this year their are no buds. I haveno idea of the variety. Part of the problem may be shade as it is in a passageway, but even those parts that have escaped over a wall into direct sunshine have no buds. The previous gardener specialised in giant brambles so i dont think its been cared for. I did put some growmore on the roots last year - but zilch.
Should i hard prune, if so how?
how do you fix such an old monster I should hate to lose it

Zero

thanks