For those that have screw on caps at the low drain point of their tank systems, here's a way of having them open during winter when the tank's full but stopping anything getting into the pipe and trapped.
Get a roll of gaffa tape[packing tape will do] and clean/dry the outside of the cap and top of the pipe. Roll the tape onto the cap so it has max contact, from one side, across the centre and onto the other side[outside face of the thread] then leave about 150mm to stick onto the top of the pipe.
Push the cap snuggly onto the end of the pipe, hold it there and then lay the tape onto the pipe and stick it down.
That allows water to drain and it will open up in a downpour.
