Plan to use roaming on my own SIM for data only. (Not bothered about calls home or texts 😄) Which is better on the pocket, Sure or Manx Telecom?

Neither. You will be ripped off with heavy roaming charges whichever one you use.
Buy a local PAYG sim and use that or just stick to WiFi

Will do! Thanks Stroller!

The IOM is not part of the UK or EU for telecoms. Some UK networks have arranged deals but most haven't.

Yeah I contacted my phone company and they said I'd be paying €1.01 per MB!! Bunch of robbers!

i am with EE and halfway over on the ferry i had a message telling me that whilst in IOM all UK & EU calls and 500mb are included in my contract .The other half is with 3 and hers is the same

Vodafone message tells me that there's now no extra charges if I keep within my monthly UK contract limits.

Interesting. My partner had a message saying no roaming charges throughout Europe and few weeks ago. He's on BT Mobile.

I'm with O2 and have heard nothing. Why am I not surprised?! Will discover situation in July when over for Southern.