I think its galvanised something or other Builder. Even quite a fine drill bit just skids off it. The patio has been down for some time and is in fine condition. I’d really rather not get a new frame installed. My problem is that I have always had drainage issues because I live on a slope and when it rains really heavily the water just sheets down the gardens, through mine and on to the next. Its been a bad year anyway but since my neighbour has some trees removed its got even worse. You suggested drilling holes under one of the bricks, leaving it loose and just pulling it out to drain the flood away, brill suggestion except I can’t drill it.
I found this one, but its round and not sure it would bear the weight of the bricks
http://www.plastics-express.co.uk/underground-drainage/manhole-covers-grates/clark-drain-chameleon-cover-26-frame-28460mm29-p-pte358 I just tried to measure the actual cover and its about 620 x 430 but I had two dogs helping me by washing my face so that’s kind of approximate. They are whatever was standard 20+ years ago and have got “Peter Savage” on them.