Have a look looks like a crack to me
View attachment 35344
View attachment 35345
View attachment 35346
View attachment 35347
Might be worth sending Mark4x4 a PM. He had his block fitted with tophat liners following a similar crack. I cant remember the name of the place he used but when he dropped off his block there were rows of blocks from some of big names who also offer the service.
I have spoken to a local machine shop in Slough and they will strip out old liner and put new one in and pressure test for 300 quid also do the heads skim both and test for 100 they have a good reputation so will set too geting the block out and stripped down -= will keep everyone updated