Is it possible, via the url link, to direct respondents to a particular page within the survey? 
e.g.. mysurvey.com/12345/Q7

Halfway through my survey respondents get directed to another site to complete an exercise on another system and I need some way for them to get back to my survey.
I was going to use an iFrame but it's looking like this might not be possible.

I don't think this is possible. Maybe someone else has a solution.

You could create two surveys and pass the Saved ID through the external tool to connect both surveys results later.

I'm just reading this post
[url] forums.limesurvey.org/forum/can-i-do-thi...ue-with-first-survey [/url]

Which seems to be exactly what I want to achieve...just got to try and understand it all!

It looks like that only works with a closed survey i.e. Tokens which I can't do.
Looks like I might have to split the surveys like you suggested.