Yes it's fine to prune it now, but the new growth probably will not show up until next spring. Getting air and sunlight into the crown by topping now, will force new side shoots next spring.

Hi, welcome to GoY, the recommended time to prune Taxus {Yew} is late summer - early autumn, they're fully hardy, can be grown in any soil from acid to limey, in any position from full sun to deep shade, and when necessary can withstand renovation pruning, Derek.

Autumn pruning should be avoided because new growth will not have time to harden off before cold weather sets in.

'Yew hedges are very forgiving when it comes to pruning, allowing gardeners to trim, shape and heavily shear with little worry. Winter is the dormant period for yew hedges, which means they are not actively growing.

During the dormant period you can trim your hedges without the worry of removing new growth. Many gardeners wait and prune in early spring, before growing season begins in summer. However, some suggest lightly trimming throughout the summer and into the fall, in addition to winter or spring pruning, to encourage fuller, bushier growth. Avoid fall trimming of newly established yews and those growing in regions with harsh winter weather, because it could spur new growth that may become damaged during winter.'

http://www.hortmag.com/weekly-tips/qa/can-yew-hedges-be-pruned-in-winter

As I said, the recommended time is late summer/ early autumn, there will be no new growth if done at this time, the new growth will come in the spring, Derek.

I know what you said Derek, I can read, but there are plenty of experts who say otherwise as I pointed out and my gardener just pruned mine last week. It's fine. "Recommended" by whom? and why?

Recommended by the R H S because it's the best time.

RHS does not say that is the best time. I'll give it to you right here:

Pruning and training
Hedges
Yew hedges can be easily maintained by trimming once in summer or early autumn

Hedge renovation
If a yew hedge becomes overgrown, it is possible to renovate it over three years. Mid-spring is the time to do this

To recreate a dense, even surface, cut the top of the hedge back to at least 15cm (6in) less than the final desired height

The following year, reduce the width of one of the sides, followed by the second side in the third year

A neglected yew hedge, with bare trunk at the base, may benefit from coppicing, cutting back the top-growth to within 15-20cm (6-8in) of the ground in early spring.

This will produce multiple stems which can be trained into a formal hedge. The disadvantage to using this method is the length of time taken for the hedge to recover, which will be several years

https://www.rhs.org.uk/advice/profile?PID=729

We're talking pruning not renovation, that was the original question, late summer, early autumn, you've said it yourself above, no doubt after googling it.
I don't need to google, I know when to prune.

I gave you the source to check it for yourself. It's not Google. lol!

"Avoid fall trimming of newly established yews"

These ARE newly established Yews as per the question.] It's OK to prune now.

welcome to GoY Debcambridge.
As for the pruning of the yews we tend to do ours August/September and last year a hard frost on the 15th Sept didn't seem to damage the yew but certainly put paid to annual salvia's , dahlias and a couple of pelargoniums.

Can I just ask why are you gentlemen are 'arguing', you both agree late summer early autumn.
the RHS rarely says 'the best time' it just states the time when plants flower/ require pruning [when it best suits the plants growth habits etc , my understanding of what the RHS is summarising in its documentation].

But plants don't read the books so when RHS says June flowering the plant may start in May or July depending on climate, soil, water etc.