  We consider an approach to fault tolerant quantum computing based on a simple
error detecting code operating as the substrate for a conventional surface
code. We develop a customised decoder to process the information about the
likely location of errors, obtained from the error detect stage, with an
advanced variant of the minimum weight perfect matching algorithm. A threshold
gate-level error rate of 1.42% is found for the concatenated code given highly
asymmetric noise. This is superior to the standard surface code and remains so
as we introduce a significant component of depolarising noise; specifically,
until the latter is 70% the strength of the former. Moreover, given the
asymmetric noise case, the threshold rises to 6.24% if we additionally assume
that local operations have 20 times higher fidelity than long range gates. Thus
for systems that are both modular and prone to asymmetric noise our code
structure can be very advantageous.
