That's probably the biggest differential I've seen, and yes I make it 30% as well. One of the problems with the lira is it volatility and for that reason exchange bureaus who hold physical currency don't like it. I think they use the crap exchange rate they always give as a form of insurance, as in we're at risk buying all this lira so lets make as much as we can from it.
The online foreign exchange places buy it on demand therefore less risk and for the consumer, generally better rates.
Finans are offering 4.155 to the quid this morning for internet banking, probably slightly less in branch for anyone not a Finans customer.
JF