Abstract: Compounds of general formula I: wherein: R1 and R2 are, independently of each other, selected from hydrogen, optionally substituted C1-10alkyl, optionally substituted —CO—(C1-10alkyl), optionally substituted C3-10cycloalkyl, optionally substituted —CO—(C3-10cycloalkyl), optionally substituted C2-10alkenyl, optionally substituted —CO—(C2-10alkenyl), optionally substituted aryl, and optionally substituted —CO-aryl, or R1 and R2 together represent an optionally substituted saturated or unsaturated C1-10alkylidene group, or an optionally substituted saturated or unsaturated C3-10cycloalkylidene group, or R1 and R2 together with the carbon atom to which they are attached represent an optionally substituted saturated or unsaturated organic ring containing 3, 4, 5, 6, 7 or 8 ring carbon atoms and optionally 1, 2 or 3 ring heteroatoms selected from O, N and S; R3, which may be the same as, or different from, either of R1 and R2, is selected from optionally substituted C1-10alkyl, optionally substituted C3-10cycl