A 42-year-old woman has a second proximal deep vein thrombosis while taking rivaroxaban as prescribed. She has no interacting medicines, normal renal function and no evidence of malignancy. Testing on two occasions shows lupus anticoagulant, anticardiolipin antibodies and anti-beta-2 glycoprotein I antibodies. She is haemodynamically well and has no bleeding. Which one of the following is the most appropriate long-term treatment?