There's mooring for about 4 or 5 boats on the tow path side (museum side) which is protected by a gate locked with a waterways key, so you can exit and return anytime.
The service block and waterpoint are on the opposite side, next to the museum shop and ticket office, and there's signed mooring space for a couple of boats there as well. During the day, when the museum is open, there's a foot bridge across the canal and gates out to the road on that side, but the bridge is swung back and the gates locked when the museum closes, so there's no way to exit, or get back to your boat if you've been to the pub / shop etc.
After posting this, and complaining last year, I was told that they would look at signage to make it clear there was no access. I haven't been back since last summer so I don't know if anything happened.
Good moorings and I'll go back. In fact might stop there next month, but was cross that the access limitation wasn't clear. I hope it has been sorted so it doesn't ruin anyone's holiday