Playing as the Welsh - shame they all sound like Dickensian Londoners and Colonel Blimp - and I'm pretty sure it's a banjo. Might have sounded like a medieval instrument if it was being played single string but there's the odd chord and arpeggio in there too.
I seem to remember an instrument in Mongol Invasion which sounded banjo-ish and actually played a line which seemed to be like (sings) 'Well I come from Alabama with a banjo on my knee' but single string and not as banjoey as the instrument in Britannia.