Welcome Ciaran
Computer network - as you say, CAT5 everywhere x Lots
Multi-room video - www.kat5.tv is good choice. Remember CT100 of even better the new foam filled coax too.
Multi-room audio - If the budget stretches then definately Sonos! It's superb.
Lighting Control - X10 DIN Rail is the budget answer. www.cbus.com is what I would fit today though if we were building again.
Heating Control - You can fit a basic commerial heating controller then upgrade later to a computer based system. Check out www.sensibleheat.com for some ideas.
Alarm System - Comfort had been the HA panel of choice for lots of us here - www.comfort.org.uk
HTH
M.