A few thoughts -
Bear in mind that pension companies / financial advisers are there to make money out of you.
Is
this the pension tracing service you have tried? A quick web search suggests there are commercial organisations who will do the same sort of thing but charge you for it.
The 'all eggs in one basket' thing - may or may not reduce fees / deductions - if they are based on a percentage and all about the same percentage, then are you going to save anything? Other than making it simpler / more convenient?
Some people argue it's worth spreading across multiple providers as the risks balance out.
It's worth looking at the terms of each scheme - some pension schemes allow earlier retirement than others so may be best left where they are (I have two deferred defined benefit schemes from past employers which can be taken at 60. I could transfer them in to current scheme, but that has retirement at 67, so if I transferred to my current scheme, I'd get more at 67, but less if I did opt to retire early. They won't be enough to retire on at 60, but might mean I could go part time.) Some schemes have a life insurance element which may or may not be worth hanging on to, others may have different provisions for a surviving partner / dependents (I don't know your personal circumstances and not expecting you to go in to detail, so don't know if either might be a consideration for you.)
At the risk of stating the obvious, transferring anything that's defined benefit (final salary / career average based) in to a defined contribution scheme is rarely a good idea.
Some pension transfers (depending on just how much is involved) require you to have consulted an independent financial adviser.
Pension Wise is a free service funded by government and may be worth a look.