Overview: This week sees the start of the 1986 British International Motor Show, held this year at the National Exhibition Centre in Birmingham. William Woollard guides you round the latest creations from the world's leading manufacturers and assesses how the best of British stands up against the opposition. Sue Baker examines the slowly reviving British sports car market and road tests the new TVR 420. If luggage space is your problem, then Frank Page could supply the answer when he compares three estate cars, including the new Renault 21 Savanna.