Do you have a tin, which should have the RAL or BS paint number, you can then get it made up anywhere that does paint mixing.
If not, try giving one of their distributors a call and ask them, OR, just buy another tin of the same :
CraftMaster say they do a 'Masons Midnight Blue' - is that the one you find too 'purple' ?
Masons - Blues – Craftmaster Paints Online Shop
Or - compare your colour to the attached colour chart (bear in mind the colour now will have faded and not be a true match to the original :
RAL Colour Chart Blue | RAL Colour Chart UK (ral-chart.co.uk)