boiler venting through pressure relief valve

I'm looking after a friends house that they rent out for short term lets.

The heating is a pain in the backside, it has two systems, downstairs ans upstairs with a thermostat on each, but neither seem to work well and guests often whack these up to 35 to try and get the rads to heat up.

I now have a new problem. The boiler tripped out, so I pressed reset and it fired up ok. It ran to two or three minutes and then made an awful clanking noise, like a very noisy valve opening and closing, then steam started venting out of what I presume is a safety valve.

Is there anything I could try before getting someone out to look at it, like draining all the hot water out, or checking valves etc?