DINERS in the territory spent a hefty $12.7 billion in the third quarter of this year, according to restaurant receipts, an increase of six per cent compared with the same period last year. In volume terms, however, total receipts fell by two per cent after discounting the effect of price changes over the period, according to the Census and Statistics Department. The rising cost of living was among the factors cited for the drop in restaurant businesses, said the Federation of Restaurant Owners. Volume of total receipts fell in all sectors with the exception of fast food shops and non-Chinese restaurants while the value of receipts rose across the board. Fast food shops registered the largest increase, up 20 per cent in value and 11 per cent in volume while non-Chinese restaurants grew 15 per cent in value and six per cent in volume. Rising rentals and a falling number of expatriates were among the factors cited for the fall in restaurant businesses. ''The loss of expatriate salaried workers is among one of the reasons for the decline in restaurant business,'' said the federation's chairman William Mark. Total receipts from Chinese restaurants remained unchanged in value but fell eight per cent in volume. Bars recorded a seven per cent drop in volume but a rise of 11 per cent in value.