11/28 cassette and you will not get a higher gear than the 11t. so change of cassette will not help much.
28/38/48 chainrings though and you may not even have to change the crankset. Just the big ring to a 50t. That may take some looking for the bolt diameter that the M151 has fitted but After Market manufacturers will have one somewhere
And not too far from you over in Eastbourne so we might meet up sometime.
And welcome to the asylum.
__________________
How long was I in the army? Five foot seven.
Spike Milligan