Shark bait......email me for my phone number if you want a chat sometime - I've done solar thermal personally and professionally. Email in profile.
You can plumb a second cylinder inline with the existing unvented cylinder but it should be done by a pro, unless you are very handy and can do it properly yourself. The viability of a standard coil used for solar depends on how much solar panel you intend to fit - most boiler coils don't have enough surface area to work perfectly with a decent sized panel.
You can use the 'hot rod' shown above if you can still get them - they don't have enough surface area to meet current boiler coil standards, let alone solar but it depends on the panel you are fitting.
If you are buying a decent solar collector, then don't scrimp on the cylinder - do it right and it will do up to 50/60 % of your annual usage.
The finances are important though - a 210 litre cylinder needs approx 14kwh to heat to 65 degrees. Oil in a well serviced boiler costs 7 or 8p per kWh (.based on oil at 60p/l and 80% efficient) so a full tank of water costs £1..05 to heat. If you use a full tank per day, annual hot water bill is £385 so you could save £180 - £230 a year.
The cost of the system will determine whether you think it's worth doing.