the power levels may be off, on the side or bottom of the router there should be a 'settings password'
type 192.168.0.1 into a browser and click enter, use that password to log into the router
then go to: advanced settings
tools>network status> click on downstream tab
copy and paste the snr and dbmv data here
If the power levels are fine, I would look at a new splitter, Labgear from Screwfix are about the easiest to get hold of at short notice...anything sold by Maplins, B&Q etc is useless for CATV