A compound of formula (I) wherein: R1 and R2 are independently selected from hydrogen, an optionally substituted C
1-7
alkyl group, C
3-20
heterocyclyl group, or C
5-20
aryl group, or may together form, along with the nitrogen atom to which they are attached, an optionally substituted heterocyclic ring having from 4 to 8 ring atoms; and R
N1
and R
N2
are selected from hydrogen, an optionally substituted C
1-7
alkyl group, C
3-20
heterocyclyl group, or C
5-20
aryl group, or may together form, along with the nitrogen atom to which they are attached, an optionally substituted heterocyclic ring having from 4 to 8 ring atoms.