A quad LNB doesn't exactly cost too much if you did need to change.

As to weather you will get a free one installed.......?

Some installers fit a twin or quad as a matter of course anyway as thats all they stock.

Most of sky's installs are sub contracted these days, so you could get anyone.  A good cup of tea and maybe a bacon sarny!

I am not sure what channels you get with the deal?  Is it not just the free ones?

There are two offerings from Sky. FreeSat and Pay Once. I believe the list of channels might be different so check.

http://www.freesatfromsky.co.uk/pay-once.aspx
http://www.freesatfromsky.co.uk/freesat.aspx

It's possible that the EPG is also different (FreeSat EPG vs Sky EPG?) I don't know but I imagine that on the cheaper Sky Pay Once deal you might have to wade through all the sky channels on the program guide, most of which you won't have a sub for. This is done so you kow what you are missing. Perhaps set up a favorites list with just the free channels.

Aside: I worked in the industry 15 years ago and it was recognised then that the company who gets their EPG into the most houses wins. Back then the thinking was that it's like a PC operating system, the more people that have your one the better. Personally I think Sky haven't exploited the capability of the EPG as much as they could. How about adverts that keep playing until you press a button on the remote to switch back to the program? That and worse is possible if they wanted.

Both Sky options use the standard Sky box but the one is £74 (Pay-Once) and the other is £147 (Freesat). The cheaper one also comes with 4 months worth of Sky packs. Seems odd?

If you are going to shell out £147 then you may as well take a look at Freesat HD. At least that way you will get a couple of HD channels and they also do the receivers with hard-drives in them now, so they work just like Sky HD boxes (although they are not as cheap as the normal Freesat boxes). They should be adding more HD channels to Freesat as time goes by.
I think places like Comet, Dixons etc do the dish and installation for £80 and then you buy whichever receiver you want to. Most sat installers will offer packages as well, look in the yellow pages.