Cheapest they've been for a long time and plenty of return flights from London, Birmingham, Manchester, Glasgow (maybe others). For departures until mid-June and for stays of 7, 14 or 21 days.
These are hand luggage only and note that they often don't include departure fees from the foreign countries (around £38pp from Mexico) so factor that in. Hold luggage is something ridiculous like £35 per bag each way and hand luggage is max 5kg. Last time I spent two weeks in Mexico I went hand luggage only but had to wear all my t-shirts when collecting my boarding pass. ;)
Still, a great deal for direct flights and I've never seen Puerto Vallarta this cheap. If you can go at relatively short notice, get on it!!