Thank you kindly for your thoughts Shep.
The attached pic is the spring tine harrow I came across and I don't think I would run this one over the rough field at all (well, maybe after a 1st or 2nd pass with something very robust and having identified/removed rocks). I've checked out the Siromer power harrow you mentioned, but bit pricey for me right now and, again, I definitely wouldn't risk that on the rough (plus, 1.3m just a bit too narrow so that would have to be the even pricier 1.7m for general tilling).
Food for thought whether a disc harrow would run over the boulders (and identify their location for subsequent removal).
Re the nature of the 2.5ac rough field: used to be grazed by sheep and then pigs were introduced for a while, but you wouldn't know it now though. Field never 'improved' (obviously for a reason) but I'm just not going to waste it - it will take trees after a bit of work. Presently riddled with bracken, gorse, bramble and grass tussocks: latter so old that they have formed solid elevated mounds. I could, indeed, bring in a digger, but again it's all extra cost and I'm in no rush (I'd rather deal with it myself, bit-by-bit, during idle moments).
In summary: I am now realising need to completely separate general tilling requirement from the rough-field improvement and look for completely different implements. Might bring a box-blade into the mix and, maybe, get a couple of longer box-blade tines (subsoiler-type blades) made up to help un-earth any deeper rocks encountered.
Thank again.