Try somewhere like
http://www.alpharooms.com or
http://www.expedia.co.uk they will be able to help with larger groups. You will probably be also offered discounts etc.
Good luck.
In fact, I´ve just checked Alpharooms and they have a weeks self catering accomodation from 113GBP in November in Costa Teguise.
TimanfayAir Car tours
cymmonn@hotmail.com
http://www.cymmon.co.uk