Day 1: Just before starting methotrexate (extra blood test) and at the end of methotrexate infusion (extra blood test) Day 2: 24 hours after starting methotrexate (routine blood test) and 20 minutes after the 24-hour blood test (i.e. just after the glucarpidase/placebo infusion) (extra blood test) Day 3+: Routine daily blood tests until the body has got rid of the methotrexate Extra blood samples will also be taken 15 days after starting each cycle and 1 month, 3 and 6 months, after starting the second cycle.