The reason that you would have been quoted different fares is that there are 2 operators between CVT and london. There is Silverlink Trains (who operate a slower, cheaper service) and Virgin (who operate a quicker, more expensive service).
The cheapest fare via London would be a Saver Return which is £37.30 going Coventry-London-(underground)-Heathrow but there would be restrictions on when you could travel back from London with that type of ticket.
The bus from Watford to Heathrow is again operated by 2 different operators. There is a half-hourly service is operated by
National Express, which is the one that costs £4 i think. The reason you were quoted an hourly £7.20 service is that refers to the Virgin service, which is timed to meet the times of the Virgin services into London. The Virgin service does run half-
hourly, at xx20 and xx50, but it's only the xx50 service which connects with trains coming into London.
The last service London-Coventry is at 2340 from Euston (see
timetable)
National Rail Enquiries announced last year that they were outsourcing 50% of calls to India (IIRC).
I don't think any of the calls are actually being taken in India yet. I worked there until February of this year, and about 60% of the people working there (in the UK) were of Indian origin, its possible you were actually speaking to someone in this country when you phoned them up.