Hi New here, so hello to all. I am to replace a kitchen mixer tap which has been in since the house was built 4 years ago. Upon inspection, the plumbing doesn't include isolation valves so I need to cut off all water to the house. Before I go ahead and fit this I want to take the opportunity to fit isolation valves or service valves. Can someone advise the best way given there's plastic pipes with a compression fitting in place? Better to add an isolation valve to the existing one or remove the old and put the isolation valve in new? Not done this before and not sure if the existing isolation nut/olive can be simply re-tightened? I posted a pic of a service valve which could do the job I believe. Posted some pics to show the existing fittings. The 90 degree fitting is on the hot feed from the combi boiler. The tap is astracast and looks like 1/2 inch fittings.