The Lancastrian invasion is on!
Getting whole-hearted Scottish backing, the Earl of Northumberland and the Duke of Somerset cross the border from Scotland into England. They swiftly descend on Newcastle, where they raise the Queen's banner and quickly gather much enthusiastic support. The city levies turn out for them, plus the Percy family's extensive holdings across the north mean that the supporters flock in with alarming speed. In the seeming blink of an eye, a large Lancastrian army has seized Northumbria and is set to march southwards.
Salisbury is of course in North Yorkshire, tasked with stopping this sort of thing. However, he has a few thousand men which is perfectly adequate for putting the metaphorical (usually!) thumb-screws on recalcitrant local nobles, but not for taking on an invasion and mass-rising by the most powerful lords of the north! Having looked over the situation, the canny Salisbury reflects that his number one priority is not dying pointlessly.
He heads south, aiming to link up with Rutland in Derby and draw on more of the support summoned up from the pro-Yorkist southern nobles, in order to have enough strength to make a fight of it.
Somerset and Northumberland have no intention of letting this come to pass, however. The agressive Henry Beaufort leads the Lancastrian host on a pounding forced-march, desperately chasing after the Yorkists as they move southwards, hoping to chew them up before they can be reinforced. After many long days of exhausting marches, the distance between the forces dwindles away and the Yorkists realise that they are going to have to turn and fight in order to buy themselves time to escape, against a force over double the size of their own.
Where, then? And how, so it isn't a walkover?
I should explain my reasoning process that took this from game-board to tabletop scenario. First of all, I looked over the places named on the map-board for a bit of inspiration. I check on Google Maps to see if there's a small town or village somewhere in the area that could plausibly have been a site (and usually Wikipedia it to see if it actually existed back in the Middle Ages.) This provides a name, sometimes even a map idea, then the general situation in the game can supply a scenario.
For this one, the map had the name 'Duffield' written on it slightly to the north of Derby - perfect spot for a stand to be taken north of a prominent town. On googling it, I saw it sits on the river Ecclesbourne just where it joins the larger river Derwent. It also forms (at least on modern maps) a little salient between the two rivers on the northern bank. This made a good potential scenario for a stand, in any case. The canny Salisbury turns a few miles before Derby and makes his stand on the riverbank, knowing that the force-marching Lancastrians will arrive piecemeal and exhausted. When they try to cross at Duffield, then he can surprise them on the banks, plus the narrow approach up to the crossing would prevent them using their numbers to good effect. There's certainly enough here to offset the numerical mismatch, which would normally rule out playing this game out normally!