I don't work for the NHS, so have no knowledge of Fleet Solutions, but...
I would contact your nearest Mitsubishi dealer and make a casual inquiry about stock availability for the model and colour you have ordered.
I had heard that the next Mitsubishi boat lands late summer (don't know how true that is), leading to September deliveries. At least then you'll know whether or not you're going to have to wait for the next boat.
I ordered my black GX4h back in January, requesting late April delivery. The bizarre response was "there are cars available in [my colour\spec] 10-14 days away, so April delivery is not a problem".
More recently, I got nervous, thinking what is the likelihood of Mitsu holding a vehicle in stock for several months?
Long and short of it is that I changed my required date just under two weeks ago from April to ASAP. It arrives TODAY (yeay!!).
It took 7 days to arrive at the dealer after being called from port.