oil & filter change every 6 months m8 - use genuine 10/40 and genuine oil filter and do the job yourself, but if you have a sludge build up then allow her to idle or a motorway journey for a good 30 minutes and let her cool down before changing the oil so the sludge build-up disintegrates back into the oil [size 32 socket for the oil filter housing]
it doesn't really help but the cts is the coolant temp sensor that helps control the timing [air / fuel] based on engine temperature, if the cts is worn then it might cause your engine to run a bit rich and over fuel thus not allowing your car to tick over properly and allow it warm up
doing low miles every day does no car any good, i just cleaned out a corsa 1.4 that did 4000 miles in a year and it was bunged up severely with mayo and crap from the breathers
for peace of mind i suggest u clean the oil filler cap and take her for a blast down the motorway for an hour and if she comes back clean then u know what it is 🙂
worth cleaning the air intake and throttle body as it will help with air flow and performance