[Dave Willis]

Several custies have yellow sills, don't know what causes it and have struggled to remove it even with upvc cleaner, seems to take a lot of effort.

I have the answer - Harpic! The blue bottle that's sold for removing limescale from toilet bowls. Wipe it over leave for ten minutes and wash off - brilliant white again. Better than anything else and effortless. Might try the stronger version - higher percentage of acid. Don't need much so doesn't seem to be any problems with spillage. Wonder if it would work on conservatory finials? Might not be good for polycarbonate.
Early days so don't blame me if you melt your customers sills 
http://www.harpic.co.uk/products/preventive-clean/harpic-100-limescale-removers/    (added some drops to pure water and it sends the tds sky high)

not played with this one yet   http://www.harpic.co.uk/products/perfect-clean/harpic-power-plus-cleaners/

Trying it on tadpoles next year.

[Smurf]

Does it melt the top layer off the plastic is that how it works? 

Interesting FAQ from website link
Can Harpic products discolour or stain the toilet bowl?

No, Harpic toilet cleaning liquids will not stain the toilet bowl. Harpic products, especially Power Plus will cling to the Limescale in the toilet bowl to highlight it. There is no ingredient in the Harpic products that can cause staining and the surface of the bowl is a sealed and not porous and thus will not absorb any liquid or colour.
 
It is important to note that Harpic toilet cleaning products are NOT suitable for stainless steel toilets. Moreover, Harpic is only suitable for use in the toilet bowl and should never be used on any other surface.

[Dave Willis]

I'll go back later see if I can push my finger through the sill.

[Smurf]

I'll go back later see if I can push my finger through the sill.

I'm sure it will be fine...fingers crossed

[Dave Willis]

I've been using it instead of Viakal for a while. Fair bit cheaper.

[slap bash]

Its made from Hypo . 

[Dave Willis]

Acid more like.

Ingredients:
Aqua

Hydrochloric Acid

Peg-2 Tallow Amine

Tallowtrimethylammonium chloride

2-Propanol

Laureth-10

Parfum

C9-11 Pareth-6

Colorant

[Spruce]

It didn't remove limescale from some customer's windows I tried it on. 

[G Griffin]

Its made from Hypo .

Isn't that cruel?
How many Hypos have to die for your to clean someone's window seals, I mean sills?

[paulben]

Harpic is that the stuff in Added Sparkle as it has a distinctive smell that I cannot put my finger on sure if Kempy was around he would tell us .
For yellow stains on sills  been using Cillit Bang with a magig eraser .