Kristell,

Just looking at a couple of sites, you can get a 7 seater Zafira for 290 euros a week here http://www.autoreisen.es/ingles/flota.php which looks a good deal compared with a couple of others I've looked at - i.e.
http://www.anthon.com
http://www.doyouspain.com/lanzarote-car-hire/lanzarote-car-hire.htm

You can hire it from the airport and drop it back off at the end of your hols so it saves on the cost of transfers as well, but if there's 7 of you there'll be 7 lots of luggage as well when doing the airport runs.

Note also, not sure if it's any use to you, but hiring 2 X 4 seater cheapo cars is quite a bit cheaper than 1 MPV.

Cheers - Ole

thanks for that. We usually only have one for 3 or 4 days in the middle of the holiday so wouldn't need to worry about luggage space! just plenty of handbag space!

Kristell, like ole has recommended i would go for autoreisen, used them last year and just booked with them again , you can just reserve with them prior to your holiday and know exactly the price and availability, you just pay when you collect the vehicle, i think they have pick-up/drop-off points in P.Blanca, PDC as well as the airport. i haven't seen any bad reports of them on here.

We've always used Cabrera Medina who are a local canarian company. Having used them on 2 of the islands, we've had nothing but a pleasent experience with them. Their choice of cars is good and they're very good at getting the paperwork out of the way & advising crucial things. They're based at the airport, and have several in the resorts

I used AutoReisen last year and will be again next month. Hiring a 7-seater Zafira for 4 days in August when you are going would cost €330 from AutoReisen which is about £230. Like Oleoleole says, it is always cheaper to hire two smaller cars than one big 7-seater if your party has two drivers to allow this option. The Zafira costs about £58 per day to hire from AutoReisen in August. A Seat Ibiza costs about £24 per day and a Citroen C3 costs about £21 per day so hiring two of either of these cars is still cheaper than one Zafira. Doubling up petrol consumption will reduce the price gap but is partly counteracted because the Zafira will do less mpg than the smaller cars.