They're reputable and have a shop in Central London but, as said, they do deal in grey imports.
I bought my Sigma 24-70 from them and, when it went wrong, I sent it to Sigma for a warranty repair. Sigma contacted me to say that it was a grey import and was not covered by the warranty. To be fair, Microglobe promptly paid Sigma directly for the repair but it's certainly something to be aware of.