Hi John - Thanks for the quick reply, + Yes I'd looked at your scheduler, and may go down that route.

I didn't realise that the scheduler piggybacks user activity - excuse my ignorance on this, but could you possibly expand a little on exactly how that works please?

Just to clarify - My custom job runs a php function that accesses a 3rd party data feed, and then writes out a copy of the data to an xml document on my server.

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Re: using a Cron - I totally agree with you, and I would usually go down that route every time. Unfortunately I am tied in to using a clients crappy/cheap/shared hosting service which doesn't support cron jobs. Just trying to get round the issue.

piggybacks user activity means that every time a user is on the website, the system takes that opportunity to run whatever jobs are scheduled (as opposed to cron which can run autonomously)
Problem with that way of doing it is that of course you can't guarantee the schedule. You want your job to run every 15 minutes but if your site doesn't have one visitor for 30 minutes, that's how long it'll take before the job is run.

@mnakaly has explained it pretty well. There are some notes in the docs page for my addon. You can also google for "poor man's cron" to find all sorts of explanations of the technique.

Thanks both - really useful!

If your hosting doesn't provide cronjobs, you may try easycron.com as a trigger.

Thanks - very useful!