Under planting a hedge?

[fabindia]

I have long runs of mainly hawthorn hedging, though it does include things like dog rose, field maple, blackthorn and elder, which seems to come from nowhere!

I normally don't cut these till all the leaves drop in Autumn but this year it has gone berserk and grown rapidly so I have started to trim them already. The problem is that although they are thick at the top, they tend to be a bit sparse near the roots.

Any ideas for under planting with something that will thicken up the base? I was thinking of something along the lines of planting holly in the gaps, it tolerant of low light levels but is very slow growing.

I could 'lay' the whole hedge but that is a big job and not sure if I am competent enough to give it a go.