Further to the above, I've been digging out prices if anyone wants to mix their own pump corrosion inhibitor. It seems that the Graco Pump Armour is a bit pricey, at around £16 per litre, so it's worth looking at a cheaper option.
For those that want to make their own "pump armour", then here's a recipe, using stuff from ebay. Bear in mind that this will produce the same concentrate as the Graco product, so needs to be diluted in the same way before use. This is actually a 60% ethylene glycol, ~1% sodium nitrite corrosion inhibitor mix, so near-identical to pump armour, and at least well within their production tolerance, right at the upper end, so it will, if anything, be slightly more effective.
Buy 5 litres of 100% ethylene glycol (around £20 from ebay) and 250g of sodium nitrite (also from ebay) for around £5 (this is more sodium nitrite than needed, but it's cheaper to buy 250g).
Mix 50g of sodium nitrite to the 5 litres of ethylene glycol and then add 3.3 litres of water and stir well. You now have 8.3 litres of "pump armour" concentrate, for the princely sum of about £25 (with enough sodium nitrite left over for four more batches). Dilute and use in the same way as pump armour. The only differences between pump armour and the above mixture is that pump armour costs over five times more, and this home made version doesn't have the blue dye and bittering agent that's added to stop people drinking the stuff. Chemically it will do the same thing, and provide the same, or better, level of corrosion protection.
If you don't want to bother making the stuff, then buy blue antifreeze concentrate (not the red or purple stuff) and use that. Any decent brand of blue antifreeze concentrate will be around 40% to 50% ethylene glycol, and will have sodium nitrite as the corrosion inhibitor. The red or purple antifreeze is not ethylene glycol, and may contain silicates and be based on OATs, so is not suitable for this purpose. Decent quality blue antifreeze is about half the price of pump armour, so still a significant saving.