Filters and flare?

Pentax lenses are noted for being very resistant to flare, but does using a cheapish skylight / UV filter (to protect the front element) make flare more likely?

Is it best to remove the filter for critical shots, or worth investing in pro standard filters to retain the pentax lens performace?

Why use a cheapish filter? You wouldn't use a cheapish lens. Good quality filters,(not so cheap)have very good coatings and don't add much or any flare to it, especcialy if you use a good lens hood. The best one is the one made for the particular lens.
My advice: Don't worry to much about the filter, but always use a lens hood.
A lens hood never does any harm, but can do a lot of good.

Horst

Try eBay for SMC Pentax filters if you really must use one when it isn't necessary.

A thin piece of glass offers no physical protection and is likely to break all over your front element, a similar impact would have probably left your front element unscathed.

I think I'd rather my £20 filter got scratched than my £700 prime lens!

If you use a lens hood How will your lens get scratched?

The basic idea of an UV filter was not to protect the lens, but to reduce the bluish tint of the UV rays. However modern lenses often have new types of glasses which don't require filters anymore. (Specially late Leica lenses). Nowadays they used just to, hopefully, protect the lens.

Lens hoods however are the best protection against flare. Take a picture with and without and you'll notice the difference. Take a picture close to a large area of green (Lawn etc) and you will find they reduce the stray light or green tinge.
The also protect your lens. They even protect your lens from a fall. At least 50% of the time.
Why? Because most lens hood are slightly larger then the front of the lens. When the lens falls, there is a 50% chance that it may fall on to the front element . The hood protects it. It may have a ding or bend, But thats nothing. Even if it falls just with a slight angle to the front lens fron the horizontal it will protect the lens. The lenshood, because of the thin material it is inherently slightly flexible and so reduces the shock.
I have a fair amount of lenses, old and new. Each one has a lens hood permanently fitted. This saves me screwing it on and off and maybe dropping it when I change a lens. It never does any harm.

Horst

Sandstorm? Not that there is much chance that I will use a camera in there, but some others might.

The main thing a filter prevents, which a hood can't, is cleaning marks. Cleaning dust or moisture off a 'sacrificial' filter means you don't need to be quite so careful, which is great if you need to have a quick wipe when out and about. I uncapped my DA15mm this afternoon, and found the front element was rather dusty, but didn't have a lens cloth on me, and certainly didn't want to go at it with a corner of my T-shirt.

TBH this filter vs no filter debate seems to be six of one and half dozen of the other. Maybe we should restrict the discussion to the issue of 'do filters cause flare'?