The key to remember here is all to do with the winter water table level.
If your tank is deep, there is a chance your drainage field could be below the level of the winter water table (therefore useless). If you have a pumped outlet, you can have the field 300mm below the finished ground level and remove any doubts about winter water table level.
Or go gravity, and have an NRV on the drain-field and have an overflow tank, so that in the event your drainage field is under-water, the outlet of the PTP goes into an ancillary tank, that can then be pumped out whenever back to the drain-field when the water table drops.
I have been on sites where one winter the water table was -3m, the next winter it was minus 200mm. Sandy soils are worse for high water table as there is no resistance to it (although offer low vp).