Southern (pre-GTR) used to use some agencies as quasi recruitment companies, recommending to certain temporary staff that they apply for various jobs in the relevant areas. Other agencies used to seem more in line with the stereotypical ideas of a temporary workforce whom the company had little to no interest in employing permanently. Indeed I am aware of a number of instances where former agency staff were taken on as permanent employees in the last couple of years before GTR - and funnily enough this includes one or two conductors. It was a different but still quite interesting picture with the extensive, pretty much in-house option of First Rail Support providing temporary & part-time staff used by FCC for work such as replacement buses and crowd control. FRS staff generally seemed less well-briefed but maybe that's because I generally came across them around the South end of the Southern network, whereas perhaps they were more familiar with Thameslink operations around central London and the MML. Nowadays there seems to be real variety in which agencies are used, some of which I am less familiar with, but others remaining which are longstanding, such as Ontrak. I note that one of the better ones, ATA, has recently made a comeback, from what I can see.
Since the formation of GTR I have noticed there seems to be somewhat less engagement with agency staff working around the Southern network, though I have to say that those staff I've come across have been no more or less committed and knowledgeable than under the old Southern franchise, when evened out and accounting for natural variations. Until the blue-vested "security" staff appeared (most of whom would appear to actually be agency customer service employees in security uniforms, from what they've told me), it also seemed that if anything, slightly fewer agency staff were being employed in non-safety-critical roles, though I don't have any numbers to hand. What I have noticed is a number of agency staff employed over the last year or so to dispatch trains on at least a medium-term, but seemingly not permanent, basis - for example, a member of staff who dispatches 8 & 10 coach 171s on Platform 1 at Hurst Green will work a reasonable shift and may be needed for up to 5 days a week at the moment, yet despite the fact that those formations are likely to remain for quite some time, some folks are not being employed permanently to dispatch them. Whether the eventual plan is to roll out conductor/driver monitors (as appropriate) when funds and approvals allow, or whether it harks back to the old policy of using agency work as a sort of vocational pre-interview test for permanent employment, remains to be seen. I have my doubts about the latter.
Southern and agencies do seem to tend to work together quite well to weed out those who don't make a positive impression, but for non-safety-related roles it seems harder to gather evidence, though monitoring always seems to me to have been fairly tight. It's certainly worth making comments (good or bad) to any station management you see, or via customer services, as although it may not seem like there is much of a response, it is actually taken seriously. I know some people will have bad experiences and I know that for anyone who's had tales to tell of the legendary behaviour of certain agencies in the North, this might seem far-fetched. What can I say? Some people give the industry a bad name when it's trying to cover work in the ways so prescribed by the franchise, or the bean-counters, or the HR policies, or at specific times of demand, or...!
With regards to the OBS role, I'm afraid I can see a time when, if the DfT so agree, GTR might want agencies to cover certain trains. This may be down to a number of factors; more rather than fewer conductors leaving than expected, and a low take up of vacancies due to people of the right calibre not being attracted by the company's public reputation; requirements of internal/DfT-specified audits for how often a train can be boarded without an OBS being present to assist, or for announcement quality, or for revenue collection, etc.; you get the picture. I have no doubt that some sort of overall competency will be needed to undertake the role, just as agency dispatchers have to have role-specific training and location-based inductions. I have no idea what the entry criteria would be, but I am pretty sure that things like "a good standard of English", probably quietly screened by subtle means of things like telephone conversations and "induction" receptionists, would feature. But there'll always be some issues which slip through the net and if this is not a safety-critical role, can we reasonably expect detailed psychometric analysis to always take place? Sadly, probably not. The real challenge will come when resource managers and other grades hardened by years of dealing with crew come up against agency OBSs who don't respond in line with protocol. If they are sceptical they will say so, if standards are not met they will highlight it, and if the OBSs can be turned away from duty, they will be. So it may be less easier than just borrowing some bloke off STM, or G4S, or anyone else.