The enlarger on the left seems to be a Meopta Opemus-5. I used one for years and the condenser head is the same for 35mm or 120 roll. If I am wrong and the enlarger is a Meopta Axomat, then it would only do 35mm though.
The problem you are seeing is probably because you are using a 50mm lens with the 120 film. Find yourself a 75mm or 80mm lens and it should work fine.
Great, cheers Martin. I think it is a Opemus-5, so I just need to track down an 80mm lens. Had a quick look on ebay and they are £65..ouch. Might double check through all the draws in the darkroom first!