Alex is perfectly correct in suggesting that U-1023 was the other U-boat in the Mersey in 1945. This U-boat left Portland on 25 May to begin a tour of various ports on the west coast of the UK, while U-776 toured east coast ports.

U-1023 was at Liverpool from 26 June to 1 July, after which it crossed the Mersey to Birkenhead where it was displayed from 1 – 5 July. It then continued its tour northwards, finally arriving at Loch Ryan on 14 August.

It may be of interest to mention here that many thousands of people visited U-1023 during her tour, but the numbers at Liverpool and Birkenhead were disappointing, partly due to the fact that another U-boat had visited previously and had already been on view, although not open to inside viewing. That other U-boat was U-532, which had arrived in Liverpool on 17 May and stayed until the 24th when, still with her cargo aboard, she was escorted to Barrow.

Thus the answer to the original question posed by Cameron back in August is that the two U-boats in the Mersey in 1945 were U-532 and U-1023.

Hope that helps.