Filling borehole brine circuit of GSHP?

[Technomad]

Newbie question here: our borehole-driven GSHP has started tripping out with a low pressure warning in the brine circuit - the latter has fired a litre or so of transfer fluid out of the blowoff (reason unknown) so needs the circuit topping up. Here in the highlands, at the best of times it's difficult to get a heat pump engineer and, during lockdown, impossible. So I'm thinking of buying the kit I need to flush and top up the brine circuit - anyone able to suggest what I might need?

TIA...