There was some confusion on installation of the Seasonic. The case has a grate on the bottom as the original PSU is mounted at the bottom of the case and the fan was facing downward.
So I put it in to match the orientation of the original PSU. Then I consulted the Seasonic install guide and there is a switch on the back of PSU for hybrid fan or normal. The guide says if I am using the PSU in hybrid mode it has to be mounted upside down with the fan facing the motherboard for thermal reasons. So a quick swap round to upside down and we were all done.
Fortunately the case has holes to fit the PSU either way which was a relief.
Tim
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