Sterling since 2010 has dropped from ~$1.6 to ~$1.4 which I wouldn't call 'plummeted'?Inflation has a role to play in this, along with the Sterling plummeting in recent years.
I expect its more to do with this. Diesel is not sort after anymore, small turbo petrol engines and hybrids are.This is entirely to do with people ditching diesel, but not enough petrol cars to go around.