on a visit to heathrow yesterday morning 27R was being used for departures and 27L for arrivals. however every 10 minutes or so an aircraft landed on 27R - is this usual procedure ?
You don't say what timeframe you are referring to, but what you saw is fairly normal.
Firstly, the Cranford Agreement no longer applies, but even when it was in force it only affected easterly operations.
Landings on the departure runway are standard between 0600 and 0700 local, yesterday morning there were 21 of them. After 0700 they are not as common, but there is provision for up to 6 per hour if arrival delays have built up - yesterday there were 13 between 0700 and 1000.