* Patients that are attending clinical appointments will be approached to ask if they will take part in the study.
* Patients will be assessed at two separate visits.
* Patients will be randomised to either exercise or rest group on their 1st visit. On the 2nd visit patients will be asked to participate in the alternate group to what they completed on their previous visit.
* At each visit they will have 2 blood samples taken, at time point 1 and at time point 2 (T1 and T2). These will be taken at least one hour apart, each with a period of at least 15 mins rest in the sitting position prior to sampling.
* On one of these two visits the patient will undergo exercise testing between T1 and T2.
* In addition, a further laboratory testing tube will be taken for a delayed NT-proBNP. This will be subjected to gentle agitation (laboratory rotator) for 48 hours prior to processing to simulate, in a controlled environment, the effect of transport and delay on sample measurement.
* Retrospectively information will be collected regarding the patient's disease severity and any change to treatment that occurred between the two visits.
* A total of 40 patients with be sampled with an interim analysis after 20 patients.
