I just did some searching and it appears that the likely cause is the clogged tube from the valve to the intake manifold. Second choice is possible a sticking EGR valve. I have not had the pleasure of working on that so I don't know the best way to go about it. I will look in My Haynes manual but I'm not sure the Brits have that system installed.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra