HONGKONG businessmen trading with China will find it cheaper to set up over the border, following the effective official devaluation of the renminbi. But the value of repatriated profits will fall as the authorities have allowed the official swap rates on the Shanghai and Beijing markets to slump. On the Shanghai swaps market - where the RMB is officially converted for trade purposes - its value tumbled yesterday following the liberalisation of the market. At one point the currency plunged by 21 per cent to an all-time low of 10.17 to the US dollar, while in Beijing it fell 20 per cent. Moves to liberalise trading in the renminbi on the mainland do not mean that a full flotation is imminent. Officials in Beijing said yesterday that there was no timetable for a freeing of the currency, and any moves could be five years away. Although the latest move will effectively make imports into China more expensive, the authorities regard it as an opening-up as it makes it easier for traders to buy currency. ''The relaxation means that enterprises short of foreign exchange can now acquire foreign exchange at the swap centres. This is tantamount to further relaxation of import control,'' said Mr Li Zhongzhou, a senior trade official involved in the GATT negotiations. Liberalising the renminbi on the swaps market could assist China in its negotiations to join the General Agreement on Tariffs and Trade. In recent talks in Geneva, China promised the GATT partners that it would be unifying its exchange rate system - and the liberalisation of the swaps market can be regarded as the first stage in that process.