Update. The SF A class we got came on a Hornby class 55 chassis. The model came with a basic Hornby decoder and it had a 'grinding' noise at low speed. I have just replaced the Hornby decoder with a Lenz standard 8 pin decoder and the 'grinding' noise is gone. Loco runs quieter and smoother at low speed and at least CV2,3,4,5,6 now all work. The Lenz decoder cost €12 despite RRP of 17.
I've decided to standardise on Lenz for non sound decoders and Zimo for GM sound decoders (141/181, 071).