can my landlord stop me from adjusting the thermostat in my flat?
Yes, in some circumstances. Check your rental agreement.
A few examples: If the cost of heating bills is included in the rent, then yes the landlord can restrict you adjusting this in a
way that would cost more so long as the temperature in the flat is maintained at reasonable levels (there is a lot of detail about what that would be).
If the flat is shared, or the heating system is common to more than one flat with only one control, then the landlord can stop you changing this in a way that would make other tenants uncomfortable.