If the paths sound and no waste pipe running under pop in these.
https://www.wickes.co.uk/Bolt-Grip-Post-Support-Spike---100-x-100-x-750mm/p/151560
Don't buy the drive in tool. Use a piece of 3 x 3 post 2 inch taller than the fixing hole and put a piece of wood on top of that wider so you...