Issue with air in radiators after solar/hot water panel installation

We had solar panels and a hot water box installed during the summer, and as the weather got colder found that the water was not staying hot.

The installer visited and fitted a non-return valve as it turns out the solar hot water box was sending hot water to the central heating system.

At some point during the troubleshooting/diagnostic phase we ended up with air in half the radiators (seems to be concentrated to one half of the house). Bleeding the troublesome radiators is futile as it comes back almost instantly.

Other than an expensive call out from a plumber, is there anything else we can try?

Thanks!