Computer back on line after check - not nearly as dusty inside as I thought it would be although leading edges of CPU cooler fan covered with a sort of greasy dust (!) that needed scraping off. Fitted an extra fan on the outside of the case to blow air into the CPU cooler duct, this fan runs all the time (very quiet 12v) so keeps a flow of external air to the CPU even after the off or standby switch is pushed and reduces the rise in CPU temperature as the internal CPU fan shuts down. Core temps now about 5º cooler although cooler weather is playing its part.