HSBC / Bruton Knowles Survey Valuer vs Landlord

Other

HSBC wanted my building (one block of 3 x 1 bed flats and 5 x 2 bed flats) revalued (not due under loan) because they say the government want them to operate 'slotting' - whatever this is. The valuer report gives a yield of 11% but a low property value. I don't know where an 11% yeild can be obtained in any industry but I may be proved incorrect with this. However, I have Savills residential rental market report which shows their findings of a 6% return outside London is acceptable. Does anyone know of an authorative source which would give a common yield as acceptable - hopefully not 11% as obviously I am going to challenge the valuer. Thanks in advance if anyone has anything to contribute.