Rare one of two error 10p coin 1975

This is my 1975 error coin that has been in my family since 1975, the counterpart error coin was kept by my nana and this one by my dad as they where attached in the change from the post office in Manchester. I don’t know where the other is now unfortunately as my nana passed and didnt let me know where. 
 please let me know a estimate on value. 

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I wasn’t trying to $ell at All. I purely want a valuation so I can compare and make the right call on hopefully keeping it. That’s all. Thank you sorry first time user  

Benelli

I wasn’t trying to $ell at All. 

Benelli

my 1975 error 10p I call it two new twenty pence 

worth much? Selling 

https://en.numista.com/forum/topic28647-4.html#p1056861 

That’s just me pu the ? In the wrong place.  
should be worth much selling? Meaning value 

simply put all I am after from this is someone to tell me a idea into what it’s actually value is before I part with it, I actually don’t want it to go as how often do these come up this clean and lined up. 
 

I will state as clear as I can. This is not a selling post in any way, you can’t buy it. 
I just want advice on a reallistic  valuation 

The coin is in great condition  and a very clear error coin.  It is a real shame you don't still have the other coin it was attached to.  I would estimate about £100.  Here in the US the larger coin shows often have professional graders present.  If it was my coin I would take it to such a show to get a definitive valuation.