I wish to get broadband internet for my parents spanish holiday home
in Tenerife, Canary Islands. They have a phone line, but only are in
the country for a few 1-week periods in a year, it wouldn't be
economical to get a standard adsl subscription which costs ~?20 a
month. If there was something which was more pay as you go orientated,
or very-low subscription fee, they'd go for that. Basically I want a
good way to give internet access for when they are there

Perhaps something on this page will help.

Staying Connected
http://www.etenerife.com/connect.htm

Most of the spanish ADSL providers require a mininum contract period of 12 months.
However, there are some providers that remove this clause and you can
cancel your subscription whenever you want without a penalty.
The most important provider offering this service is Wanadoo
(http://adsl.wanadoo.es/adsl4mb/). Once you have requested to sign-up,
an usual period until ADSL working is around 15 days.
If any other assistance is needed, please tell me.
Regards.