Thanks Brian. Due to the flow rate I get from rising main (c30l/min)
I'll probably have to set my store higher than 65degC (or choose a
bigger PHE - I've been looking at options which have a heat load of
100 and 104kW) to achieve the necessary output.

On the secondary/output side can anyone help with my remaining query.
Here it is: I have decided I will set my TMV (located just after the
PHE secondary output) to 55degC. My question is how do I calculate/
estimate what secondary output temperature I need to have to achieve
55degC where 1 hot tap/shower is open at the same time? Obviously,
if there was only ever ONE outlet open, the secondary output could be
designed to be 55degC at the designed max flow rate - easy. Equally,
if I had an excessive secondary output temp, then there would prob be
plenty of "headroom", so the TMV may always be mixing with cold, even
if 1 hot water outlet is open. However, this isn't a very efficient
sitn really as you'd either need a large (expensive) PHE or heat your
store to a really high temp. I would much prefer to "tune" the system
design to be able to design the minimum required PHE secondary output,
at the same time ensuring that my shower doesn't cool down if the 2nd
shower or the kitchen hot tap is turned on.