Those GSF ones are suspiciously cheap. Possibly 1.6 FSI. GTI ones are £160-£185 from Autodoc for Boschers. As Clarky said, considerably more from VW.
Unfortunately an injector swap is a grand in parts these days when you factor in gaskets, seals, fitting kits and what not. Another 4 in labour if a garage is new to the job. Seasoned experts can probably do it in 2 once you have the knack for getting the intake off. I'd rather shove wasps up my arse than do that job again.
Do all 4. Doing 1 is a false economy as the others will follow suit. Running the engine for extended periods misfiring will kill the cat converter.