Interesting that your greenhouse plants are more affected, so i think that excess heat stress is perhaps one cause. But also the main time of BER happening is at mid-maturity when plants already have some fruit and need lots of nutrients esp calcium to make the second and third truss, so your tunnel toms may not be there yet?
It seems that main cause is lack of calcium from soil too dry OR too wet. Adding calcium to soil or spraying on leaves is reckoned to be ineffective. I wonder if I may have been overwatering in that Homeacres soil appears to drain less readily than Lower Farm soil. I should have twigged that, but to be sure I am buying a soil moisture meter from eBay for £15, will let you know how it works!