Try Using a strong pond liner on top of the concrete. But make sure there are no sharp edges in the concrete before you lay the liner.
i know we had to replace our liner in our pond this year.

I'd go for a pond liner too. We used old wool blankets [from the charity shops] as a cheaper fleece underlay under the butyl liner of our brick walled raised pond.

Does anyone else think this pond is excessively deep?

Good depth for some of the larger waterlilies and big koi, but a hazard for small children.

No its not too deep it is just the right depth., for the size you have got.

My hubby has just made our pond bigger and has gone down exacty 1 metre

I will say this if you do have small children about in the garden just keep an eye on them.
We did our pond when my 2 children were13 years and 15 years old.And they are now 18 and 21 years old.
Mind you when my neighbour had his raised pond my son used to hang over the side to look at the fish but someone was always there to keep an eye on him and NO he didnot fall in.
So basically use a bit of iniative., where children are concerned.
But recently my hubby thought he would make it larger.

Actually I was thinking more along the lines of the biology of the pond...? Tho thinking about it, mine's about 2.5ft deep and that was only just enough this winter.

They say a toddler can drown in 3in of water...