@GeriL The Glasgow postcode is serviced by Two Major suppliers, one being OR (Openreach) and the other is CF (City Fibre) they are seperate and will therefore have different speeds and profiles on the Network, and sky uses Both off them now.
One can be working fine the other maybe not so, speeds do go up and down depending on the Network load, so although you say that you have a FF300 or above to at least get your speed quoted earlier then with the connection do not expect to get that totally every time you check it is just not possible, there are far to many variables for everything to be the 100%.
As an example i have a FF500 connection, the lowest that i have seen to date during what is termed peak time is sustained 200Mb/s, it is enough to cover the needs so nothing major, when it is quite i always hit 500/75Mb/s no issues both on Ethernet/Wireless testing, but all off my Equipment works and importantly i do know how it performs.
