Now that seems strange to me unless this tap has never been used since you were there last. I would be investigating it further.i went round last nite to explain to her and turned hot tap on and brown water came out for a few secs....could not have timed it better....but did she accept what i showed her.....no.
Has there been any other work done there lately? Has there been an underground burst service pipe, water main or have the water board been working nearby? Sometimes fire brigade test the hydrants and this stirs up a load of dirt.
I can think of only a couple of ways that a boiler can create its own crap and send it into the hot water supply
1 If the mains pressure is at only say 2 bar then expanding system water could force its way through a faulty plate heat exchanger. A rare fault and even rarer on a ravenheat.
2 The hot water has been connected to the heating system. [Yes this does happen and I can show you a place now where some nugget has connected into the heating system for hot water!]