- only those channels affected?
not sure; a quick scroll it seem no others are.
203 on now is badly affected and it affects BBC3 or CBBC
204 is BBC4 / CBeebies not affected
200 (BBC1) also but not 201 (BBC1 HD)
- do the interferences happen at a certain time (e.g. evenings only) or throughout the day?
for 203 constantly
- how long have you been having this NZ feeling?
:-) it is just the digital interference pattern I use to see when we had heavy rain