How old is your DC now OP? I did have a look but couldn't see that you'd mentioned it.
And how long did you take for mat leave - and would you take the same length of time again?
Just wondering whether a very slight delay in going for your second child coupled with a long mat leave might cover the bulk of the childcare and only need you leaving a nursery for two children for a short period.
The only question I would have is whether you've planned for after school care and holidays? If nursery places are so extortionate in your region then I imagine after school and holiday child care is going to be pretty hefty too.
I think the issue is whether you can ACTUALLY afford it, but are just nervous about such high costs - or whether it could cause financial difficulty/leave you without any financial resilience?
I know you want two DC but with the situation you describe - working in such an expensive area, no affordable childcare, no local family who can support (presumably), no do-able alternative options such as changing jobs or moving home - honestly, I think you have to consider whether having another child is actually viable.
I know the nursery fees issue is only short-term, but you need to be sure you can make it through those years unscathed, plus afford the ongoing childcare for school holidays, after school etc for two children. Only you know how this could affect your finances.
FWIW, I have twins and it absolutely killed me financially because there's bugger all extra help for multiple births. You have my sympathy as it's a tricky one. But sometimes it's worth stopping to consider whether having another child could actually jeopardise the stability and security of your family. And what if you fall pregnant with twins? The risk increases as you get older. What if the second baby has a disability/health condition and needs more care than a regular child - can you afford that? My twins both have a disability so I speak from experience. You never think it will happen to you, until it does.
Sorry to sound so negative - I just have been through the process of desperately wanting another child before realising that actually, it really wasn't fair or practical. And that's a very hard decision to face up to. Might not be the case with you, but it's worth thinking about.