Cannot use a certain number on excel

Need someone's help please!

I have an excel spreadsheet that has a master tab and then sub tabs where the data then feeds into the master automatically by a formula. I have just added some new data on to the master and the sub tab and inputter the formulas but for some reason out of the 14 addresses I added with unique property numbers, excel is not allowing me to use 1 of these numbers, i can type any other 8 digit number in and the spreadsheet will link but as soon as I use the number I need to use the 2 sheets stop speaking to each other. They all have the same formula as it has just been copied and all other cells/rows/columns are updating. 

The only way to have them linked is if i do a -8 at the end, if i try to just put the 8 normally it stops linking the sheets and its driving me mad!

Thanks

Emma