installation help

Was going to use the spare a780 i have and put upstairs for the kids,running a cable feed is no problem but how do i get the internet feed to it as a long way from my router which runs my downstairs box and also will the fact i am splitting feeds even more impair my signals and connections

Home Plugs are best, they use the electric circuit in the premises.
Ethernet cable from router to Home Plug which is plug into electic socket direct, a power surge or electic splitter can disrupt the signal

Ok ,homeplugs seems the way to go,now i have a splitter at the moment for router and cable feeds so will i be ok to split again for new cable feed

Yes OK to split the cable again using a Konig 6 port splitter no problem.
Get one out of Argos if it don't work for you take it back, if it does work OK take it back and get one cheaper elsewhere. lol