I have solar thermal installed with the HP in 2019 and it works a treat. It is actually supplied via 6 hybrid panels from Solimpek. As the PV part operates it produces lots of heat as PV panels do. This heat is collected from the rear of the panels and transfers to the Mitsubishi 300ltr DHW tank via the dedicated solar coil at the bottom. The solar circuit is pressurised and is operated by a Grant solar pump station and controlled by a Resol controller. The HP works perfectly on the heating circuit and only steps in to heat the DHW as required, which for 8 months of the year is rarely. 

@abernyte This is the first time I've heard of hybrid panels and they sound like a really neat idea.

So your setup uses a DHW tank that specifically has a solar coil for the solar thermal. I on the other hand, have only a standard DHW tank installed with no dedicated solar coil. My question is: Would I therefore need to replace my existing DHW tank with one that has an additional coil for solar thermal - or - install an additional dedicated DHW tank for the solar thermal alongside the existing DHW tank used by the ASHP?

I think having two DHW cylinders is cumbersome and leads to other problems to solve such as how to manage legionnaires cycles and so on with the heat pump.

I suspect that the cleanest solution is to use a DHW cylinder which is designed with the solar coil as it will also have the required pockets for the top and bottom sensors. I am sure it could work with a separate cylinder but that does sound a bit cumbersome unless you had a utility area with plenty of space. I use a Mitsubishi solar cylinder with an Ecodan HP and FTC5 so that it all speaks to one another.

I don't use the legionella cycle and it is switched off on my system.  The DHW is supplied via a plate heat exchanger and there are no dead legs in the domestic plumbing. The solar thermal keeps the tank sitting at 60C for much of the spring and summer so I feel that the risk which was already very low to be vanishingly so.  

If the hybrid panels have a downside it is that they do not generate as much PV as a conventional PV only panel of the same size but that was a trade off that I accepted as I use just about all of the output on running the total heating system.

I'm beginning to think solar thermal is something that makes sense for those with high DHW demands and maybe not something that is necessary for a typical 3 bedroom house.

Looking at my own heat pump data (3 bedroom house), DHW only accounts for approximately 1/5 of the energy usage. Once you've made the transition from gas to renewables with a heat pump, there's really not much benefit going further with the solar thermal.

There are solar assisted heat pumps which are ingenious as they capture all the benefits of solar thermal for heating and DHW. But they are unique systems that use a large grid like collector panel which is mounted on either the roof or a wall. There's only one company that makes and installs them and if that company went bust, you'd be stuck with a white elephant if you happened to have one.

Since you already have the ashp it makes little financial sense to add solar thermal to it. We already had solar thermal (vacuum tube type), so had another dual coil type cylinder fitted. The installer said he finds many ashp customers already have solar thermal (makes sense early adopters all along). 
We are very happy with the combination which on a sunny summer’s day gets to 54C without running the ashp. So our DHW ran about once a week (the rare rainy day). 
But if we hadn’t got solar thermal we wouldn’t have it put in now. 

@judith That makes sense to me!

If I was in @alex_n 's shoes, I would look at solar PV + battery instead of solar thermal. 

I was originally contemplating a solar thermal installation last year. However, I decided in the end to install solar PV instead together with my heat pump. What I have found is that, on sunny days (i.e., when solar thermal would be useful), the solar PV can generate more than enough energy (and, if the sun is shining when the domestic hot water is being heated, power) to heat all the hot water in the cylinder with the heat pump with plenty to spare. Spare electricity can be exported or used to charge a battery. Spare hot water from a solar thermal system cannot be used. Solar thermal systems are probably only useful in cases where you can have a high confidence of using all or nearly all of the generated hot water most or all of the time.