A few thoughts:
"I have recently received inheritance,..."
Anything from that inheritance that you may wish to specifically leave to your children should remain in an account solely in your own name. If it is placed in a joint account then that will automatically pass to your DH regardless of anything you put in your will.
"...I don't want them to just piss it all away when they turn 18 😳 so it's just earmarked in my head for them just now. Maybe that's not wise though??"
It is certainly possible to specify an older age in a will when they will inherit, for example 21 or 25. Your solicitor will explain the details of that.
"I'm not sure whether to just put that my estate all goes to DH, or would I specify that he gets a % and each child gets a %? I do trust DH but part of me thinks if it's not written down then the kids might not get the inheritance I want them to get (as who knows what might happen in the future, in a worst case scenario!)"
If you read some of the threads here you will see that the "worst case scenario" does actually happen from time to time.
Just speaking personally, I would suggest maybe changing the ownership of the home to tenants in common rather than joint tenants (ie you each own 50% of your home rather than owning the home jointly) and then leaving your 50% share to your children with a life interest for your DH to live there.
And, of course, he has a mirrored will saying the same thing.
This ensures that at least half of the family home is left to the DC and will also protect half if one of you later goes into care. Again, your solicitor should explain this in detail.
"Alsos I want to put a 'catastrophe clause' in, in the event that me, DH and kids are all obliterated at the same time. How many beneficiaries do you think would be OK to put in, in that event??"
As many or few as you want, it really doesn't matter. It could be your brothers and sisters or it could be your nephews and nieces etc. You can also name charities if you wish.
Generally speaking, if your children also die at the same time but they have children of their own (so, your grandchildren) then their share will be left to your grandchildren.
My understanding is that if you do not include a "catastrophe clause" and all beneficiaries are dead (and don't have children of their own) then the rules of intestacy apply, it is referred to as a "partial intestacy".
So, in the situation you mention, everything would be left to your parents. If they were not alive then any of your siblings would be next. After that comes half-siblings.
From there it would go to your grandparents. After that it goes to your aunts and uncles or then your cousins.
After that it goes to any half uncles or aunts (eg if your grandparents remarried and had children from the second marriage - although, they don't actually have to have been married of course)
If none of the above are alive then the estate goes to the Crown.