Where is 12 Grimmauld Place?

[Catlady (Rita Prince Winston)]

Did anyone notice that the subject line said 21 Grimmauld Place, 
leading me to wonder if someone had a numerological theory on 
something else being hidden on that block.

--- In HPFGU-OTChatter at yahoogroups.com, "bluesqueak" <pipdowns at e...> 
wrote:

> Roughly, but it would take about 20 minutes to walk from Charing 
> Cross Road to Kings Cross. Kings Cross Station is on the Euston 
> Road, 

Where is Euston Station, which JKR admitted is actually the one 
that has the barrier she envisioned the wizards crossing to get to 
Platform 9 and 3/4? 

> and has a neighbourhood around it (called 'Kings Cross'. I 
> think it's the station that's named after the neighbourhood.)

I seem to recall two years ago someone educating us stupid Americans 
that King's Cross and Charing Cross (chere reine cross) are both 
named after crosses some Plantagenet king (I forgot which one) put up 
in memory of his late wife.
 
> 12 Grimmauld Place could also be in the Kings Cross neighbourhood, 
> which has definitely seen better times. It's currently one of the 
> most notorious red light/drug dealing districts in London, with the 
> local residents fighting a bitter battle to try and get the 
> authorities to clean the place up. If 12 Grimmauld place *is* in 
> King's Cross, I can understand why the kids were only allowed out
>  of the house with an adult ;-)

How does that affect users of the train station?

> But personally, I would go for Kings Cross itself, (snip) has a 
> park  called 'Joseph Grimaldi Park', which may have suggested the 
> pun. :-)

Any idea after whom 'Joseph Grimaldi Park' is named?