> could you reduce the ones you have added for this PR?

No, this is currently supposed to be generated from metarelate, although that is [under discussion] (https://groups.google.com/forum/?fromgroups&hl=en-GB#!topic/scitools-iris-dev/twa74txl408).

> Should the secondfixedsurface type always be interpreted in this way?

That follows a precedent set [elsewhere] (https://github.com/SciTools/iris/blob/master/lib/iris/fileformats/grib/load_rules.py#L328), and has caused no known problems so far.
It _is_ allowed to have different surface types, such as 'surface' and 'height level',
so we might have to address that sometime but that's not required as part of this PR.