Now we've hopefully shown the OP why unvented is a bad idea, lets answer the question.
Simple and safe option is to do away with the pressurisation setup and convert to open vented - which is all a standard cylinder is designed for. Small F&E tank which just needs to be above the highest point in the system. Either connect the ballcock to the water supply, or you can manually fill it with a hosepipe if needed - connected to mains is far more convenient.
Whether you add corrosion inhibitor is an interesting question for this system. No steel rads, pipe all copper or plastic, probably only the pump liable to rusting. If you do add inhibitor, then I'd suggest you find the manufacturer's site and find out what the dosage rate is supposed to be - the instructions on some packs are worthless for situations like this (and in my case, with a thermal store). For example, I works out that in my flat I needed 3 or 4 litres of Aqueous Logic inhibitor to get the required concentration.
As to when you want to connect it to the house, the logical option would be to use an indirect coil in the cylinder and connect that to the house system - going indirect means you can keep the water separate rather than having one big system. Use an S-Plan setup with the office cylinder as a separate zone. I'd be looking to use more than one cylinder stat - so you can bring on the system if the cylinder is getting close to "empty" (heat wise), but otherwise only bring it on to "top up" when the house system is already on. Maximise efficiency by adding the office load when the house system is already running. Actually, it'll probably need a relay or two - but I haven't really though too much about it.
Of course, if you might ever have plans for the office and house to be in separate use, then you'd want separate heat sources so each gets it's own bills. Letting something "heat included" is often a recipe for waste and high costs.
Simple and safe option is to do away with the pressurisation setup and convert to open vented - which is all a standard cylinder is designed for. Small F&E tank which just needs to be above the highest point in the system. Either connect the ballcock to the water supply, or you can manually fill it with a hosepipe if needed - connected to mains is far more convenient.
Whether you add corrosion inhibitor is an interesting question for this system. No steel rads, pipe all copper or plastic, probably only the pump liable to rusting. If you do add inhibitor, then I'd suggest you find the manufacturer's site and find out what the dosage rate is supposed to be - the instructions on some packs are worthless for situations like this (and in my case, with a thermal store). For example, I works out that in my flat I needed 3 or 4 litres of Aqueous Logic inhibitor to get the required concentration.
As to when you want to connect it to the house, the logical option would be to use an indirect coil in the cylinder and connect that to the house system - going indirect means you can keep the water separate rather than having one big system. Use an S-Plan setup with the office cylinder as a separate zone. I'd be looking to use more than one cylinder stat - so you can bring on the system if the cylinder is getting close to "empty" (heat wise), but otherwise only bring it on to "top up" when the house system is already on. Maximise efficiency by adding the office load when the house system is already running. Actually, it'll probably need a relay or two - but I haven't really though too much about it.
Of course, if you might ever have plans for the office and house to be in separate use, then you'd want separate heat sources so each gets it's own bills. Letting something "heat included" is often a recipe for waste and high costs.
