* FTSE 100 down 0.4%, FTSE 250 flat

Oct 6 (Reuters) - A firming pound drove UK's exporter-heavy FTSE 100 lower on Tuesday after a three-day run of gains, while Restaurant Group surged after it forecast upbeat sales numbers.

The blue-chip index dropped 0.4% and lagged its European peers as the sterling rose above $1.30 per dollar level, with investors scaling back bets on UK interest rates turning negative.

Midcap stocks were flat, as gains in consumer and industrial firms offset losses in healthcare companies.

Frankie and Benny's owner Restaurant Group gained 8.4% after it unveiled improved like-for-like sales numbers for the period since the start of July.

Premier Oil jumped 13.4% after it reached an all-share merger deal with oil exploration and production company Chrysaor. (Reporting by Shashank Nayar in Bengaluru; editing by Uttaresh.V)