YY to Asda prem range for super tiny, they do 3lb, 4lb and maybe 5lb, my twins were 5lb6 and 4lb1 at birth, down to 4lb9 and 3lb9 after a few days. Asda 4lb stuff was still big but the vest and baby grows have poppers on the shoulder sections (vest), the bottom for nappy changes and all down one side (vest) or legs so they're brilliant to get on and off too, wish there were more ranges like it! .
Boots, Morrisons and Mothercare also do lovely early baby/tiny clothes.
Pampers micro nappies were good for the first 2 months, they're sized up to 5lbs but mine wore them for much longer, found them hard to get though, Boots was the only place we found them.
I bought my son's clothes from mothercare and next. He was 4lb when he was born and he drowned in them. Staff on the neonatal unit ended up putting dolls clothes on him lol. Eventually 5weeks later he was allowed home weighing 5lbs he just about fitted into the premature baby clothes I had bought for him. Quality wise next was best (personal opinion)
John Lewis do early baby and tiny baby and is where most of 5 week prem DS's clothes came from. They are happy for you to return and change for bigger sizes/refund if you get it wrong! Also had stuff from next, boots and Tesco.