Solar hot water - Pre-heater

I have a perfectly good hot water service at home so I wont be throwing it away just yet. But I think I've decided on what I'll do instead. My plan is to make a pre-heater to heat the water that goes into my hot water service, so less energy is required to get my water up to temperature.

I already have a working understanding of heat exchange principles, so I'll leverage my new knowledge and attempt to apply it to pre-heating my hot water inlet. By doing so I'll avoid having to make a solar collector that is capable of taking mains (in in my house mains-like) water pressure.

Currently my hot water service takes delivery of its water at around 13 deg C and then uses fossil fuel to heat it to something like 65 deg C. If I can collect some heat and store it in an insulated container, I should be able to make a simple heat exchange device to transfer that heat to the inlet water of my hot water service. If I can spare my hot water service the trouble of even a few degrees It will have been worth it. But I'm hoping to contribute more like half the energy required to make my showers hot. If I can do that I can feel a bit more confident that I can make a replacement hot water service when my existing one gives up one day.

I guess that means my official target for my inlet water temperature is a 26 deg C increase from 13 deg C to 39 deg C.

That should be easy enough in summer but it's moving towards mid-winter here so I will be very pleased if I can get even close.