Excluding cost considerations, would there be any objections to installing say 3.6 sq. meters of panels rather than the calculated 2.4 sq. meters? The extra cost would not be that much and hopefully I would benefit from extra free heat transfer during the spring and autumn months. Comments welcome.

Nick - thanks for your reply. Yes, evacuated tube (the Apricus 30). (Am also considering the Darkline T2 F-P). We are thinking of a 180 - 200 litre thermal store which it seems would fit well with your rule of thumb figures. Do you agree with my philosophy or would I just be wasting money on a larger panel(s)? It just seems to me that a modest increase in the initial capital outlay would be recouped by additional energy savings in the long term.

Good practice is to size tank to daily DHW usage and then size collector to match tank size.
Rule of thumb for evacuated tube is 75 to 100l per sq m.
Less than this will give extended periods of stagnation when the tank is at max temp and the panel will heat up to 200degrees plus.
During stagnation the fluid breaks down and needs replacing more often.

From practical experience of my panel I would optimise what you can get during the summer months and anything you get in the winter is a bonus.
Levels of solar radiation in winter are very low and not economic - my tank is rarely gets upto anything other then tepid in the winter months.

I would spend the extra on energy conservation measures.