"there are some very poor quality teachers about."
Absolutely. I think that is true of all jobs / professions, isn't it? There are some poor doctors, poor nurses, poor accountants, poor managers, poor plumbers, poor checkout operators etc etc etc. That is not at all to say that they are in anything other than the small minority, but they are there.
Locally, the real horror stories about poor teachers, in terms of classroom control and subject knowledge, come from a single sex superselective in the next town. Other local schools of all types do have some poor teachers, though not in such large numbers - either those who are poor across the board, or who are poor for a certain group e.g. those who find it specifically hard to deal with lower groups, or to stretch the most able.
In primary, it is common for teachers to be stronger in some subjects than others - I will freely admit that my art and PE lessons, though competent, are limited, especially when it comes to extending the most able .