Tried renewing brake and clutch fluid. On the clutch bleed nipple I used a normal 9mm spanner. Both the ring end and flats slip and the nipple won't undo. 8mm just won't fit no matter what. Unsuccessfully tried encouraging it to start moving with some needle nose pliers. It appears to be made of plastic. 

 

Would a 9mm brake spanner work better? Is it likely to need a new slave cylinder? Any suggestions welcome. Thanks.

The one on our Fabia is definitely plastic, but 11mm across flats I think, so a bit more grabbable.

 

I'd be tempted to leave it. Although it may well be mucky, unless you have reason to believe air has got in there I'd be surprised if it wasn't fully functional.

The slave cylinder has a relatively benign working environment without extreme temperature, so even if there is a local slight water content, it's very unlikely to boil and cause trouble.

As above, I'd be tempted to leave it alone unless it really needs it. If you do think it needs doing, give it a spray and leave it alone for a while. Patience is a virtue.

 

You should be able to get a little 6-sided socket in there without a lot of bother, ideally shock it with an impact driver than swinging on it.

 

Make sure you've got a spare bleed nipple to hand if you do decide to take it out. 

I would never use an impact driver on a plastic bleed valve, probably why I've never ever changed the clutch system fluid along with the brake fluid over the past 20 years and 6 VW Group cars. Some VW Group cars have the plastic fluid vent valve fitted to a short section of pipe - and I can think of a very good reason why they do it that way!