Hi new to this forum. Can anyone advise me on the potential use of a tank in tank hot water cylinder which has a combined water content of 750 ltrs, 300 ltrs being domestic and 450 being primary, being connected to a ground source heat pump and solar panels (6m2, which were speced to heat just 300 ltrs). My concern is will the panels be sufficient to get heat into the domestic (the coil for the solar is in the bottom). Comments appreciated

It depends on how much domestic hot water you use, and how much sun you get, and to a lesser extent, the ratings of the solar coil in the tank. The capacity of both the tank and the size of the panels would be sufficient for a typical 'merican family of 3-4 in most of the US, with US-style water use. But without knowing your latitude & weather details and a bit more about the tank it's hard to say if it works as a system or not, but assuming a reasonable solar designer specified the panel size for 300liters, it's probably good.

If the total thermal capacity of the tank is larger than normally used for that panel area it's not a disaster, quite the contrary: The solar will in fact run slightly more efficiently due to the cooler average panel temperature. The bigger concern would be if the thermal capacity of the were too small, leading to overheating issues and lower solar operating efficiency. The difference in standby losses of a 750 vs. 300 liter tank are quite small (vanishingly small, if comparing a 750liter tank running an average 55C to a 300 liter tank running an average 65C) or compared to the panel-efficiency hit you suffer by running the solar at higher-than-needed temperatures. Assuming the geothermal loop is designed to maintain the top of the tank to 45-50C the overall efficiency should be pretty good.