We have techs who leave their base and can drive for 30mins to reach their workplace for that day. I'm looking for members' views on how we would stand if we insisted that, having provided them with hand cleaning equipment and chemical toilet facilities, they should have their lunch break in their van. Otherwise their time away from their workplace could be 90mins.

Many of us are peripatetic workers and spend all day away from the office/base, it's a matter of assessing what conditions your staff are exposed to:
Here in the Wildlife Trusts we send staff and volunteers out for short days to do practical work on nature reserves, where they have little in the way of facilities bar fresh water, hand cleansers and a mini-bus to shelter in. But everybody knows what to expect, the days are short, if anybody feels not up to it they don't go and if the weather gets bad they just come back to the office. If you expect staff to do longer days in more difficult conditions then more facilities should be provided, in a previous post we offered a lunch allowance if staff were more than 3 miles from office at lunch time. Sufficient to cover a cafe lunch where they could use facilities.
Hope that helps a little?

I was trying to establish if they're expected to drive for 30 minutes when they get caught short?

I should have pointed out that their workplaces can be on forestry commission land and areas such as this i.e. not close to any facilities. Toilet arrangements can be addressed by issuing chemical packs e.g. http://www.traveljohn.co.uk/#.
The main discussion is in their rights should they insist that they eat at the base (30mins drive away) or whether the van can be classed as suitable.

What do you mean by 'hand cleaning equipment'?