I have never used one of these but having spoken to one or two people here in the UK who have, but do not own, they gave favourable reports. Suggestions as to what /which to buy. I am looking for something I can mount on the Encore for quick acquisition, as in driven boar/deer etc., but which I can quickly dismount and replace with a normal scope. Your views would be appreciated.

Highly suggest you look at the Holosun 507c, with its dot within a circle reticle. This one is a hot seller in my neck of the woods. I got to shoot this Holosun vs. a standard red dot, and would not own the latter if the former were available. The Holosun reticle is that much better, for me at least.

https://holosun.com/index/product/detail/id/104.html

Whoops. When you said Encore, I was thinking handgun as in the United States. I would still search for a rifle optic with that reticle.
Oh, and welcome to AR.

For use on a handgun I've not found anything better than an UltraDot 30, as seen on my BFR. They're light weight, and the tube seems to "acquire" much more quickly than a reflex red dot. It should be even quicker on a rifle. With 30 mm quick release rings you should be good to go.

ditto on UltraDot.

UltraDot also. Have held up for many years on my .500 Smith and .475 Linebaugh FA

I have ordered a Holosun 507G ( green dot) for my Encore rifle .500-110 ( Same as mild 500NE) Have not got it yet but as bill said they are very well liked.

Also, that UltraDot 30 looks excellent. Nice rings too.