Keep the trunks to use as leverage. Dig round and sever as many roots as you can. Saw, axe, mattock, whatever you can find. We removed 100 feet of conifers like that. I used a chain saw to cut roots, but it is probably better to use a reciprocating saw.

Thank you Owdboggy. I will have a go tomorrow. Think it’s going to be a long slog……

failing that cut them off as close to the soil as you can and add SBK [designed to poison the roots] and let the chemical kill them.

I'd go for Owds method personally but if all else fails......

Never had any success with SBK and root killing. Nor have we managed to get the conifer stumps here to begin breaking down with the stuff which is supposed to hurry the process along.
Viburnums do not usually have major sized roots nor do they often have deep tap roots, so getting them out should not be too traumatic.
Watch your back though!

Want to plant the bed up in autumn so rather not want to use SBK. Guess it’ll be down to manpower…..

I do the same as Owdboggy. I am still able to get things out this way and female and getting on,Keep going round cutting roots as you go using stumps to lever it this way and that. in the end just the big root or an annoying small one! People make the mistake of cutting things to ground level, instead of leaving enough stump for leverage.