Activities do not need to be carried out free from pain ... ?
When DWP say this in their ESA decisions what are they relying on?
Possibly CIB/14587/1996 - “It is all a matter of degree”.
I don’t think they can rely on CIB/14587/1996 because there are a number of descriptors (e.g. descriptor 1) where the test is one being able to do then task without significant discomfort or exhaustion. Discomfort of course encompasses pain.
I also don’t think CIB/14587/1996 is any longer good law because in AS v Secretary of State for Works & Pensions (ESA) –  UKUT 0587 (AAC) – CE/1470/2013 Judge Wikeley held at 
21. Within the legislative scheme as a whole, this principle only makes sense in the context of the needs of a modern workplace and the level of activity that an employer attuned to the requirements of disability discrimination law can reasonably expect. Plainly, the test is not about a high-pressure working environment, e.g. a call-centre with demanding targets or a factory production line with a fast-moving conveyor belt. Equally, however, the test is not about what the person can do in their own home and entirely in their own time and at their own pace, subject to no external constraints or pressures whatsoever. If reasonable regularity is judged by the latter criterion, then the test has ceased to be a test of “whether a claimant’s capability for work is limited by the claimant’s physical or mental condition” within regulation 19(1) of the ESA Regulations
The then Mr Commissioner Rice held in CIB/14587/1996 that the descriptors were to be looked at in the context of daily living.
ESA decision makers often make rather formulaic statements about the “modern workplace” so we need to see those statements in the light of AS
But what is a ‘modern workplace’ - a steel mill, a quarry, a building site, a port, a corner shop, a big 3 supermarket, a bus/lorry/taxi/car/bicycle/train, warehouse, grotty back street factory/office, a plush new factory/office, an advice centre. They are all work places and they are all modern in the sence that they exist (haven’t gone the way of coal mines) and people work in them.
Whilst I agree that the specifics of CIB/14587/1996 (i.e. are no longer good law in an ESA context I think that the general theme of it being “a matter of degree” is consistent with the general conclusion in AS (which in turn is approving an earlier decision in AH) - “The words ‘repeatedly’, ‘significant discomfort or exhaustion’ and ‘reasonable timescale’ are normal words in everyday use…I am not going to attempt to define what these words mean”.
I would suggest that an ‘everyday’ understanding of the phrase ‘significant discomfort’ would be that there are some levels of pain that are not so severe that they constitute significant discomfort. So I think that the DWP are correct in what they state. The corresponding argument from a claimant would be to submit to the tribunal that on an everyday reading, significant discomfort is less severe than significant pain and that from this one might conclude that most pain should constitute significant discomfort.
As an aside, The area that I do think AS (and in particular the paragraph you mention) is very helpful on is as authority that the ‘account of a typical day’ that we often see recorded (claimant washes and dresses independently, cooks themselves a meal, speaks on the phone to their family etc etc) is not particularly strong evidence for what a claimant would be able to do in the context of the workplace.
I think you are right and there is authority for your argument..
When I cite AS I also go on to say
I am reminded that Schedule 2 descriptor 1(a) (mobilising) provides
Mobilising unaided by another person with or without a walking person stick, manual wheelchair or other aid if such aid is normally, or could reasonably be, worn or used.
1 (a) Cannot unaided by another person
(i) Mobilise more than 50 metres on level ground without stopping in order to avoid significant discomfort or exhaustion;
(ii) repeatedly mobilise 50 metres within a reasonable timescale because of significant discomfort or exhaustion
The descriptor clearly provides that any mobilising that can only be achieved at the expense of suffering significant (not necessarily severe) discomfort is not to be taken into account. This was confirmed by Mr Commissioner Stockman in the Northern Ireland decision CK-v-Department for Social Development (ESA)  NICom 28 C17/12-13(ESA) (at [11-12])