Silent processing failure on GeoField: exceeded memory limits?

Hi guys, this looks like a very well put together module. Your attention to detail is obvious, and much appreciated.
This is my first time to use the module and I'm wanting to import a taxonomy with geographic terms.
First, I set up the taxonomy and added the fields required. Then I added one sample record with values. Then exported this using taxonomy_csv, and I inspect the resulting export file.
This process flowed smoothly: no errors reported by the module during taxonomy export.
However, the csv export file contains no values in the fields downstream of the geofield containing WKT: polygon.
Looking at the data exported, the WKT field simply stops in mid-string, so I reckon this definitely points to some sort of resource limit.
However, the failure is silent.
Looking through your help files, I noticed the recommendation to increase server and PHP memory...
I can't easily increase memory to the entire VPS: it's already got 750Mb just for this website and there's no traffic/load at present so it's all available.
Both /etc/php5/cli/php.ini and /etc/php5/apache2/php.ini are set to 256M max memory
So the concern is that this appears to be failing with just one record with a modest WKT polygon (100kb).
We deleted the WKT polygon, re-ran the export and all fields come through fine.
So:
1. Do you agree this looks like memory issues?
2. What are the reasonable limits for the module to manage?
3. Are there any strategies for handling the bulky fields separately?
Any advice much appreciated,
JB