I would have a look at the tensioners immediately if not sooner. It sounds like the pads are wearing thin, and if they break up its bye bye engine. Pull the passenger side cam cover and check to see if the chain is wearing a groove in the pad. Its a pain, but it only costs a couple of hours labour and a cam cover gasket.
If the pad is ok (unlikely) then its the actual tensioners that are failing and ironically, thats less serious since they only rattle on startup rather than potentially destroying the engine. It turns in to a 4-figure job then though since the tensioners are >£500 each.
You may find, of course, that it still rattles after changing just the pads and in fact it was the tensioners all along. I had that with one recently which was rather frustrating. Its a £1000 gamble whether to just do the pads and hope it'll be fine or whether to just go for the tensioners and guarantee you only have to do it once. For me, I'll do the pads first for £20, but then its not a huge deal for me to have to do the whole job again. Annoying, but its less than a day's work. If you're paying a high rate for labour then it makes diving straight in with the complete tensioners less of a gamble I guess.
Find out what the tensioner pads look like and go from there
