Hi,
You don't need to split the cable, run one of your ethernet cables from a socket behind where you modem/router is currently up to the hub (switch is better) in the loft and just connect the two. Everything else can hang off of the hub as you described.
For the loft I'd use (in your case) a 10/100 switch say
http://www.ebay.co.uk/sch/i.html?_sa...0fs108&_sop=15 or a gigabit (still use cat5e for the cable)
http://www.ebay.co.uk/sch/i.html?_nk...p=15&_osacat=0.
Regards,
J.