I bought in Amazon five Faithfull sash clamps a pair of years ago. I retrofitted them following Paul’s recommendations and yes, they made better, but they keep being bad clamps. Yes, they are light, they are “relatively” cheap and dont require the investment of these made-for-rich-people-clamps, but they aren’t clamps for a solid work, at least in my opinion. About the Silverline brand, all tools that I have tried from that brand were, with no doubt, junk. Trash. Wasted money.
I prefer the classic english style sash clamps heads, combined with a timber of laminated wood as beam for extra strenght. I use these (the green ones at the end of the page), bought at Fine Tools, in Germany. Just one thing: for a mysterious reason, the same clamping set that I bought for 16 euro/set, costs almost 43 euro now. I shouldn’t probably buy them for this price, but with a laminated timber of 1 inch thick, they are strong as a rock.