Easy to do at yourself at your local transport office.
You need photocopies of your passport data page, entry stamp and current extension, photocopy your IDP and home country licence as well just in case.
If applying for bike and car you need 2 separate medical certs, you can get these from any clinic.
A residency cert from immigration or a work permit with your address on it.
Take all of this to the transport office, you will have to do some simple tests, brake test, matching the 2 sticks test and colour test. The first time you will have to watch an hour or so of accident videos and after that they will take your photo and print your licences, the first ones are valid for 2 years, subsequent ones are valid for 5 years.
I wouldn't bother with an agent as you will still have to supply all the documents required and it really is pretty easy to do it yourself.
Thailand doesn't recognise different cc categories, there is no up to 125cc here, you either have a full motorbike licence or you don't have a motorbike licence at all. Also you won't have the motorbike stamp in your IDP unless you have a full motorbike licence.
Everybody knows how insurance companies love to pay out and they will do whatever they can to avoid it, this has caught many people out in the past as the insurance company won't pay out if you are not correctly licensed.
Thailand doesn't recognise different cc categories, here you either have a full motorbike licence or you don't have a motorbike licence at all, so basically you can't legally ride a motorbike in Thailand and as you are not legal your insurance won't cover you.
The first stamp is an under consideration stamp, normally the immigration visit you during this one month period. The second stamp is your year extension stamp, your 90 day report will be 90 days from the date of this one
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/23, normally you will have a separate piece of paper stapled in your passport with the date to report on it.
You can transfer from your UK account to your BKK bank account directly, the cost was £15 per transaction, this is how I transferred large sums from the UK to here 7 amounts ago.
If you are at the Poipet border you may as well use it rather than spending money going to another border.
Worth noting is if you have entered Thailand twice by land on visa exempts this year you will be denied entry and have to go to Phnom Penh to either get a tourist visa or fly in on a visa exempt, this applies to all borders not just Poipet.
You don't need to enlarge it as enlarge means make bigger and you don't need to make it bigger, you need to renew it.
It is pretty easy to do yourself, just go to the transport office with your passport, your 2 year licence, a medical certificate from a clinic, you need 2 if you want car and motorbike, residency certificate you don't need this if you have a work permit with address in it, and copies of all above.