Hello!
I am attempting to pull a total sum of numbers associated with each of these tasks as a grand total from one project into another.
e.g. the values in the 6th field (3,400 and 250) equal to £3,650 which I then want to pull into a grand total column in a tracker.

Welcome, @Jenni_Dent,
I don’t think you can easily just get the sum alone to appear in another project.
Other ideas as workarounds:
You could get the sum to appear in a Universal Dashboard chart (if on Advanced or above plan, I believe).
Or you could “roll up” the sum into a portfolio that the project is contained in.
You could multi-home (add to project) the set of tasks to be summed into a section in the other project such that the sum, along with the tasks, would appear there as well.
Thanks,
Larry