Hi Tinto and welcome to GoY, sorry no photos attached to your question.

no photo ,but i can tell you when, now or in the spring would be a good time.

Prune the hydrangea around late March, not now. Leave the dead flowers on to provide frost protection to the lower buds

you usually prune a mop-cap type hydrangea in late spring, removing the dead flower-heads down to next pair buds. if there is any damaged/diseased wood then that can be removed now if you want. If the whole plant is too large then remove a third of the stems this year and then another 1/3 the yr after. Many hydrangeas flower on last years wood so if you prune it out now there wont be flowers next year.

To get bigger flowers, cut them all the way back. In late winter or early spring, these shrubs can be cut all the way back to the ground. Smooth hydrangeas will produce much larger blooms if pruned hard like this each year, but many gardeners opt for smaller blooms on sturdier stems.

As Moon Grower says we need to know what kind of Hydrangea it is. H. macrophylla flowers on old wood so is not pruned much. H. paniculata flowers on new wood so is pruned in February as hard as you feel is needed.

Hi, welcome to GoY, without a photo we don't know which of the 80 species of Hydrangea you have, so we will have to presume that you have a plant of the most common species, H macrophylla, most species of Hydrangea are pruning group 1, which means they should be pruned in late winter or early spring while dormant, but Hydrangea macrophylla is pruning group 4, which means cutting off the dead flowerheads back to the first bud behind the flower, on larger plants you can remove a 1/4 to 1/3 of the stems, to encourage new growth lower down the shrub, this should be carried out annually in early to mid spring, don't cut all the branches back, or you will have no flowers next year, because they flower on buds formed the previous year, as Sbg says above, if you want larger flowers, then give them afeed of a balanced fertilizer, Derek.

I've not heard the term smooth hydrangea before WB. what aspect is smooth?

I haven't heard that term before either WLB and look forward to your responding.

Smooth ones are arborescens, in which case you can cut them to the ground. No idea why they are called that. My one arborescens is Annabel and if you do cut it tall the way to the ground the stems are not strong enough to support the huge flowers. I compromise by cutting about two thirds off. The flowers still weigh the stems down though and it doesn't seem to affect the size of the flowers at all. And in any case they throw up a lot of new growth from the base.

Tinto if your hydrangea flower has a round head like a mop it will most likely be a macrophylla. Arborescens are less readily available. If it has flowers shaped more cone shaped like lilac it is most likely a paniculata type.

Hydrangea arborescens known as smooth hydrangea, wild hydrangea and sevenbark, it is a species of flowering plant in the family Hydrangeaceae. It is a small to medium-sized deciduous shrub up to 3 m tall that is native to the United States.

I have H arborescens 'Annabel' but haven't heard it given a common name. Wonder why sevenbark?

Are you able to up load your photo then we will be able to offer better advice.

Well we definitely need a photo of the other plant to give an id :)

Derek will tell you I'm an inverate looker up so I looked in the complete Oxford dictionary we are very lucky to have. Sevenbark was listed but sadly no derivation given so I asked Google, and it says its because of its many layered bark.
Must remember to have a look next time Annabel gets pruned!

cant really say I have noticed the bark being particularly layered and that's the sort of feature I tend to notice.

There is a H. arborescens called Incrediball which is an Annabel type but with stronger stems so it is not supposed to flop.

Have seen it advertised OB, but too late - no room for another one! (Unless this one gets the chop...I've rather gone off it now.)

We have one or ten H. paniculata types, know what you mean about running out of room Trouble is they keep producing more of them.
We also have Red Annabelle which is also a bit of a weak grower.

Wow, I've never seen that one - sounds spectacular (if you prop it up!) Yes, I wasn't prepared for the way it wants to spread itself.