do I need to address open pen risks.

Perhaps not have to, at least as far as BS 7671 demands, at least for a single caravan at home. The section 708 requirement for TT'd supplies for caravans only applied to caravan parks, camping parks and similar locations - and the definition of a caravan/camping park is land that contains two or more pitches. The ESQCR prohibits the connection of a caravan to PME (9(4)), but the requirement only applies to a distributor, not a consumer. All of which probably explains why the usual way of connecting up a caravan at home is via one of these...
https://www.aandlleisure.co.uk/uk-mains-adaptor-cable-13a-to-16a-fly-lead-hook-up-caravan-generator----016118a-2987-p.asp

That's not to say that as the designer you can't decide that it might be better to take some precautions though. It's not always simple though - TT in a domestic curtilage can sometimes be challenging - finding somewhere to bang in a rod that's guaranteed to avoid underground services, be far enough away from PME'd cables (esp PILC ones) and bonded underground metalwork not to be unduely influenced, and then keeping the entire TT'd installation out of reach of PME'd ones.

Another option might be an open-PEN device .. but BS 7671 doesn't particularly recognise them for use outside of section 722 (EVSE).

   - Andy.

do I need to address open pen risks.

Perhaps not have to, at least as far as BS 7671 demands, at least for a single caravan at home. The section 708 requirement for TT'd supplies for caravans only applied to caravan parks, camping parks and similar locations - and the definition of a caravan/camping park is land that contains two or more pitches. The ESQCR prohibits the connection of a caravan to PME (9(4)), but the requirement only applies to a distributor, not a consumer. All of which probably explains why the usual way of connecting up a caravan at home is via one of these...
https://www.aandlleisure.co.uk/uk-mains-adaptor-cable-13a-to-16a-fly-lead-hook-up-caravan-generator----016118a-2987-p.asp

That's not to say that as the designer you can't decide that it might be better to take some precautions though. It's not always simple though - TT in a domestic curtilage can sometimes be challenging - finding somewhere to bang in a rod that's guaranteed to avoid underground services, be far enough away from PME'd cables (esp PILC ones) and bonded underground metalwork not to be unduely influenced, and then keeping the entire TT'd installation out of reach of PME'd ones.

Another option might be an open-PEN device .. but BS 7671 doesn't particularly recognise them for use outside of section 722 (EVSE).

   - Andy.