Self-installations cannot be grid-connected. The MCS-accreditation is required to do this (think of Gas Safe for heating systems). Proper certification is required when dealing the export tariff applications.
If you want to power the main property in addition to the summer house, I can only recommend getting a proper grid-tied installation done with the additional expense of proper installers. How far is the summer house from the main property? Distance affects final output.
Seven-years is correct regarding payback, (more with battery storage), but ultimately down to consumption levels.
If your happy just powering the summer house only, then a standard standalone kit (think of canal boats with solar panels) may be an option?
Hope this helps.