I heard this will be sorted out very soon (I got a Humax box sometime ago) so looking forward to it
Do I need a double LNB or quad LNB? I'm sure the installers will probably know anyway, of course at least I want to be able to record one channel and watch another etc.

You need a twin but for a sky dish you won't find one. And anyway quads are much the same price for all dishes so might as well have the 2 spare outputs

I think they're going to set up a new dish as we looked on our Sky dish and it just had a twin, so not enough spare outputs.
So another dish with a twin (or quad) would be fine I guess Not sure what the installers will bring with them.

If you have a dish and you are happy with it, then for Sky and Freesat you should only have 1 dish. If you have a dish with a single or twin they should upgrade that to a quad and leave you with one dish. You can get octo lnbs also.

For just Sky and Freesat I would find two dishes an eyesore.

If there is something wrong with existing dish though, get them to take it down and replace it, still no reason to have two dishes.

A valid reason to have two dishes is if you need to receive several satellites and cannot accomplish satisfactoirily with multiple LNB arm or motor*.

* Note that HDR doesn't properly support a motor though.