Unless you see flames or an impact do nothing. The vast majority of reports from the public are false alarms and simply waste a lot of people's time and effort.
If you do see 'something' then ring 999. But don't expect anything that looks like a joined up response. I've had the police (on one occasion ALL the police within a 30 mile radius) at the strip several times.
Just a landing aircraft is often enough for a 'concerned member of the public' to call. Inspite of the police knowing the strip was there and having my numbers they all turn out looking for overtime or a simple bit of sight seeing.
If you're in the aircraft then use the radio - ATC are much better at getting a response organised and sending it in the right direction. Twice I've been rung up by police and ATC enquiring about missing aircraft. The ATC queries are as you might expect - G-ZZZZ have you seen it, the police are dressed up in we are looking for a yellow and blue which may have, could have, do you have any information...... It's is just the culture of the two organisations - but I know which one I would rely upon.