If memory serve hebes are short lived perennials if you've had them for 20 years then they have probably reached their end. If you are living in the area of En gland that had the severe drought last year this could have killed them off too.

I was thinking that 20 years is about right for a Hebe. if you can try and get a last cutting now then it is time to buy new.

They have now been removed & I am quite excited to look for other shrubs to take their place, although sorry to see them go. I had no idea that hebes had a finite lifespan. Also, they were on a slope so any drought conditions would have exacerbated their water shortage.
Thank you so much for your helpful comments. Regards.

That's it look to the positive, a chance to have new shrubs in the garden! Just for info many shrubs have a life span of 20 years of even less... some of the Daphne's for example.