CREDIT demand from would-be shareholders in Shangri-La Asia pushed the overnight interbank rate up nearly two percentage points yesterday morning, but no funds were injected into the banking system by the Monetary Authority. By midday, overnight rates had risen above four per cent but they slid down during the afternoon to close at 3.375-3.25 per cent, compared with 2.375-2.25 on Wednesday. ''Everybody was trying to get funds to subscribe for the shares and the interbank rate did go up. It's come back down but we're watching the situation,'' said Joseph Yam Chi-kwong, chief executive of the Monetary Authority. Applications for the issue closed yesterday, with a substantial level of oversubscription expected, even higher than the 50 times level forecast earlier in the week. Demand for credit to apply for shares on margin has caused big problems in the interbank market in previous issues. The January flotation of Denway investment sent interbank rates above seven per cent. As a precautionary measure, $650 million was injected into the system on Monday. Cheques for the issue were cashed last night.