My mustard leaves are pale yellow with cupping and the new leaves look worst, is this sulphur?

This is most likely sulphur deficiency. Mustard and other oilseeds are heavy sulphur feeders, and a shortage gives pale yellowing that shows on the younger upper leaves first, often with upward cupping of the leaf margins, thin spindly plants, delayed flowering and poor pod and oil filling. The key way to tell sulphur from nitrogen by eye: nitrogen is mobile so it yellows the older lower leaves first, while sulphur is much less mobile so it yellows the younger top leaves first, and sulphur yellowing tends to be a uniform pale green over the whole young leaf. Even so, the two are easy to confuse, so please confirm with your Soil Health Card or a soil test before deciding. The standard correction is a sulphur source. Many farmers apply gypsum or single super phosphate, which both supply sulphur, or elemental sulphur and bentonite sulphur products. A common gypsum range for oilseeds is roughly 80 to 100 kg per acre worked into the soil before sowing, but the right amount depends on your soil test sulphur level and crop, so follow your card and local KVK advice and do not over-apply. A foliar spray can help a standing crop showing symptoms.